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ABP Southampton Environmental Statement for Port of Southampton: Berth 201/202 Works

Appendix C

Investigation of the Effects ofthe Berth 201/202 Works onHydrodynamic and SedimentProcesses

Appendix C Investigation of the Effects of the Berth 201/202 Works on Hydrodynamic and Sediment Processes

Environmental Statement for Port of Southampton: Berth 201/202 WorksAppendices

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Appendix C. Investigation of the Effects of the Berth 201/202 Works on Hydrodynamic and Sediment Processes

Investigation Methods Study Area The study area is defined as the area over which the potential direct and indirect impacts of the scheme are predicted to occur, as identified during desk-based reviews, field studies, and further informed during the scoping exercise and on-going consultation with stakeholders and interested parties. Direct impacts are defined as physical effects arising within the footprint of the capital and maintenance dredging of the berth, and disposal of dredged material. Indirect impacts may arise as a consequence of the effect of the proposed works on the hydrodynamic and sedimentary regime, for example erosion and accretion of intertidal sediments in adjacent areas, or the dispersal of sediments arising from any disturbance from the dredging works. Given the localised extent of the proposed berth works, the main study area is within the Test Estuary, extending from Redbridge to around Dock Head. In order to determine any impacts of the proposed berth works to the Southampton Water and Solent system, the actual study area extends from Redbridge to around Lymington in the Western Solent and Wootton Creek in the Eastern Solent. A more regional extent that covers Hurst Spit at western end of the Solent to Selsey Bill in the east and about 30km offshore is also considered in the assessment to cover the areas of potential effects that might arise from the relocation of dredged sediments to the Nab Deposit Ground. Desk-Based Investigation A desk-based study has been undertaken to establish the baseline conditions for the Environmental Impact Assessment (EIA), as well as to determine the extents and the specific parameters that need to be included in detailed numerical modelling studies. This evidence-based study involved undertaking an extensive literature review, extracting relevant physical, chemical, biological and ecological baseline information. Within this baseline review, coastal and estuarine study areas associated with the proposed development have been defined, appropriate to the specific element of investigation. Primary sources include recent EIAs, research papers and specific estuary studies, a number of which were undertaken for the previous 1996/97 channel deepening assessment and subsequent 10-year monitoring programme of the foreshore, along with the extensive studies that were associated with the earlier Dibden Bay proposal. Another important part of this desk-based investigation was to determine relevant gaps in information that would be required to undertake the EIA assessment. This, in conjunction with consultation with key stakeholders (Appendix A), has led to the specification for additional field investigations, which are discussed in more detail in the following section.


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Field Investigations Bathymetric and topographic information have been collated from existing field investigations and extend the evidence-base, including:

ABP Southampton hydrographic surveys - these are undertaken at least annually for most of

the navigation areas in the Port. In areas of sedimentation, additional surveys are undertaken in order to direct the maintenance dredge activities and confirm the results of the campaigns, which generally occur in the ABP controlled areas;

Channel Coastal Observatory (CCO)/Environment Agency intertidal surveys - these were undertaken by remote sensing (e.g. LiDAR) in 2006; and

UK Hydrographic Office (HO) multibeam surveys - the latest 2007 survey has provided an update of the bathymetry within the Solent.

This information has provided complete coverage of the intertidal and subtidal areas, which is particularly important for modelling and determining the hypsometry1of the intertidal areas. In addition a geotechnical site investigation has specifically been undertaken to support the EIA, which covered the berth pocket of Berths 201/202 to determine the volumes and characteristics of the materials to be dredged. This is essential information to inform parameter selection for modelling, the selection of the appropriate dredging plant and the methodology to be used. The site investigations included boreholes, vibrocores, cone penetration tests (CPTs) and surface grab samples to supplement existing information. This was supported by a series of laboratory tests on the sediments, including chemical analysis of typical samples to determine the quality of the material. With respect to the dredge requirements, this information is summarised in Chapter 3 of the main ES document. Modelling The proposed Berth 201/202 works has the potential to affect coastal and estuarine processes within the Test Estuary, and to a lesser extent in Southampton Water and the Solent. The magnitude, extent and duration of these effects is complex and difficult to predict. Consequently, numerical models have been prepared and run to allow any effects on coastal and estuarine processes associated with the proposed berth configuration and disposal of dredge arisings to be identified and, where possible, quantified. These models were also set up with the aim of assessing stakeholder concerns, as identified during the consultation process. The suite of models developed to inform the EIA allows investigation of the following aspects associated with the works: Changes in hydrodynamic and sedimentary regimes brought about by the proposed scheme; Sediment plume dispersion during dredging operations; Dispersion of dredged material from the deposit site; Potential for ‘in-combination’ effects due to interaction with other proposed developments,

including any identified beneficial uses; and Estimation of future maintenance commitment within the berth.

1 Hypsometry = The measurement of land elevation relative to sea level.


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Overall Approach The models are evidence-based, having been underpinned by the latest available datasets used during the model configuration (including the most recent multi-beam surveys) and have undergone extensive calibration and validation. This approach has been developed in line with current best practice recommended in ‘The Estuary Guide’, which provides an overview of how to identify and predict morphological change within estuaries. The guide can be accessed through the website: http://www.estuary-guide.net/ and was recently extended under a current Defra and Environment Agency Flood and Coastal Erosion Risk Management R&D Programme (FD2119), which brings together all the existing and new outputs from the Estuaries Research Programme (ERP). The numerical modelling approach has been designed to consider impacts relating to the proposed Berth 201/202 works at both regional and local scales. At the local scale, which effectively covers the Test Estuary where the Berth works will take place, and the regional wider Southampton Water and Solent areas. The models have been applied to investigate the detailed changes in water levels, flow speeds and directions and their effects on the sediment patterns within the study area. The full extent of the model is shown in Figure C1. The local model provides enhanced detail of the local study area, using an optimised curvilinear grid with a resolution varying from around 100m in the Solent down to approximately 20m in the tidal rivers. This grid representation allows the features of the Berth 201/202 pocket deepening to be resolved within the model. The seaward boundaries have been located in the West and East Solent around Lymington and Wootton Creek, respectively. The up-estuary extent of the local model includes the Test and Itchen Estuaries allowing freshwater inputs near to their tidal limits. Within the Hamble Estuary the boundary was located at the Bursledon Bridge and at the Chain Ferry in the Medina Estuary. The regional scale model has been used to assess effects of disposing dredge arisings from the berth pocket at the Nab Deposit ground. An assessment of tidal excursions in this area has determined the extent required to investigate sediment plume dispersion. The regional model has also informed the boundary conditions of the more detailed local model. The regional model covers the Solent area at a resolution of 133m, and has been dynamically linked to a larger shelf model covering the whole of the English Channel. This approach has provided a reliable description of the complex tidal patterns around and within the Solent. Figure C2 shows the extent of the regional model. The grid was centred on the Nab Deposit Ground extending more than 30km offshore and to the east to ensure that the maximum likely excursion of sediments deposited at this site would be contained within the model. Model Calibration In this section, examples of calibration are provided for the regional and local models described above. The full details of the calibration and validation process are provided in Annex C1 (at the end of this Appendix) where for each model a description of the hydrodynamic (variations in tidal heights and currents) and sedimentological conditions (where appropriate) is given along with the calibration and validation against field measurements supplemented by synthetic datasets in areas not covered by measured datasets.

http://www.estuary-guide.net/


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Local Scale An example of the level of calibration achieved within the local model is illustrated in Figures C3a and C3b in which spring tide water levels at Calshot and Dock Head are compared with measured data. At these locations, the model prediction of high water (HW) values was good, on average within ±0.01m of measured values with lower standard deviation of 0.04m at Dock Head compared to the 0.08m found at Calshot. The figure shows the distinctive double HW with the general shape of the tide curve well reproduced by the model. An added feature of the tide, which develops during the flood phase, and around two hours after low water (LW), known as the “Young Flood Stand”, is also well represented by the model. The asymmetric profile of the tide is clearly reproduced by the model, with the flood phase (of around 9 hours to the second HW) lasting longer than the ebb phase (of around 3½ hours). The modelled current speeds are shown to compare well with the field data at Lains Lake, as shown in Figure C4 for spring tide conditions. Key features of the tide are well-reproduced, including the multiple slack water periods over the flood tide and the stronger ebb flows. Peak current speeds are generally within ±0.04m/s for both the flood and the ebb. The local model has been further calibrated/validated against measured suspended sediment concentrations (SSC) for cohesive (muddy) sediments. Model predicted and field measured SSC data were compared at four locations throughout the estuary with the comparisons at Fawley and Lains Lake provided as an example in Figures C5a and C5b. The high level of variability in the field data is not replicated by the model, although mean concentration levels are similar and are in good agreement with the measured data. The gradient in mean concentration levels along the estuary, with increased suspended sediment concentrations towards the Solent, is well predicted by the model. The predicted rates of sediment accumulation have also been compared with the volumes of maintenance dredging that have been required since the channel was last deepened in 1996. At three of the five locations considered, the predicted rates of deposition were found to be within ±25% of observed values, which is considered to be a good level of agreement given the limitations of the recorded data. The model predicted annual deposition rates as shown in Figure C6 with further details of the comparison provided in Table C.1. Table C.1 Deposition rates

Dredge Zone(s) Location Observed (m/yr) Modelled (m/yr) 8 Bury swinging ground 0.313 0.2

15, 18 Berth 201 0.195 0.2 38, 39, 41 Junction Channel 0.224 0.18 44, 46, 47 Ocean Dock 0.206 0.45

58, 59 Lower Itchen Toe 0.323 0.4 In qualitative terms, the sediment model provides a realistic representation of the distribution of sediment accumulation. The model is even shown to replicate the erosion of intertidal sediments as is known to occur during the spring ebb tides. Further, by applying a scaling factor to siltation rates for the 15-day simulation period, it is possible to predict mean annual siltation rates throughout the Southampton Docks to a satisfactory level of accuracy. On this basis, the sediment model is also considered to be suitably well calibrated for the assessment of any scheme related impacts due to the proposed Berth 201/202 works.


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Regional Scale Figure C7 shows the comparison of modelled and observed water levels for three key locations. Results are plotted for a three day period for spring tidal conditions. The model is shown to reproduce many of the subtle features of the tide and demonstrates good agreement with the measured data. There is generally a high level of agreement in terms of the timing and elevation of HW and LW. A quantitative assessment of the calibration confirms that the levels are predicted within an error margin of ±0.19m, or ±5% of the mean spring tidal range of 3.8m at Southampton. On this basis the model is demonstrated to predict water levels to an acceptable degree of accuracy. In terms of phasing, the time of HW is predicted to be ‘late’ by up to 11 minutes. Conversely, LW is shown to be ‘early’ with a time difference of approximately 10 minutes. Overall, the model-predicted water levels were calibrated to an acceptable standard, particularly in view of the 10 minutes temporal resolution of the measured data. Modelled and observed currents are plotted for comparison in Figure C8 for a location in the central Solent off Cowes (Site I). This comparison was undertaken by matching the observations to model output for a period with compatible tidal range. The analysis of differences in current speed and direction shows that peak current speeds and directions are generally well predicted by the model. For the example provided here, modelled peak flood and ebb currents agree within ±15% of the observed currents. Current directions are also in good agreement, typically to within ±10°. The level of calibration achieved is considered to be reasonable. Overall, the regional model is shown to be calibrated and validated to a high standard in terms of water levels, current speeds and directions, with the rotational characteristics of the tide and characteristic features of the Solent tides resolved by the model. The high standard of calibration achieved throughout the majority of the model domain confirms that the model is suitable for the assessment of effects during disposal of arisings at the Nab Deposit Ground. Calibration Discussion The calibration and validation of these two independent models has been described in detail in the calibration report (Annex C1). An assessment of the suitability of each model for its intended purpose can be made from the conclusions provided in this section. Local Scale

- The local 2D hydrodynamic model has been calibrated and validated against available measured datasets within Southampton Water and its sub-estuaries;

- The model provides a good description of tidal water level variations, especially in terms of the first HW on spring tides;

- The neap tide water level variations are more accurately reproduced by the model than spring tides;

- Tidal currents and directions are well predicted by the model, even under the relatively weak flow conditions experienced during neap tides;

- The model is also shown to be able to reproduce characteristic features of the tide such as the flow reversal over intertidal areas, which occurs around HW and at the time of the Young Flood Stand;

- The cohesive sediment transport model provides a good description of mean concentration levels along the estuary although peak concentrations are less well predicted;


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- The model results from a 15-day simulation can be scaled to represent long-term deposition rates within areas where dredging within the Southampton Docks is undertaken;

- The model also reproduces observed behaviour of sediments such as the erosion from intertidal areas near Fawley during the ebb tide; and

- The model is suitable for the assessment of scheme related changes in both hydrodynamics and cohesive sediment transport processes.

Regional Scale

- The regional 2D hydrodynamic model has been extensively calibrated and validated against a combination of measured and predicted datasets;

- The model provides a good description of tidal water level variations throughout the Solent over a range of tidal conditions;

- The model provides a very good description of tidal currents throughout the model domain and over a range of tidal conditions as demonstrated by the comparison at fixed positions; and

- Further validation of the model, not reported here, has confirmed that the model provides an accurate description of current patterns throughout a typical spring tidal cycle.

Hydrodynamic Modelling For the modelling of change, the local model covers the study area with sufficient resolution to define the features of the berth works. The following effects of the works are presented in both a spatial and temporal context: Water levels, with respect to height and tidal phasing, particularly at HW and LW, thus affecting

tidal propagation and range; and Flow speed and directions.

Water Levels The detailed effects of the Berth 201/202 works on maximum and minimum water levels in Southampton Water and the component sub-estuaries are shown in Figure C9. A negligible reduction in HW of less than 0.002m (2mm) is predicted in the area local to the container terminal in the Test Estuary. No changes in HW levels are predicted throughout Southampton Water, and the Itchen and Hamble Estuaries. At LW, there is a negligible increase in levels of up to 0.001m (1mm) in the area of the container terminal and along the upper sections of the Itchen Estuary. It should be noted, however, that the detail of the modelling in the upper section of the Itchen Estuary is limited and, therefore, the results are subject to greater uncertainty than the more detailed areas down estuary, where no change to water levels are found. No changes in water levels are predicted to occur elsewhere in the study area. The modelling, therefore, indicates that changes to the tidal range will be confined to the area local to Berth 201/202, where there will be a maximum reduction in the spring tidal range of up to 3mm. These millimetric changes are so small that they are considered to be insignificant with respect to potential impacts to the intertidal habitats of the study area.


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Flow Speed and Directions Spatial (map) plots have been produced to show the baseline flow environment, and locations where changes in flow speeds and directions are predicted to occur as a result of the proposed berth configuration. These changes are very localised and so the plots generated cover the area from Redbridge to around Dock Head. The plots have been extracted for times of higher flows that exist on the flood (HW -1.5 hours) and ebb tides (HW +4.3 hours) for the tidal conditions at Dock Head. These times represent peak flood and ebb flows within Southampton Water and the Test Estuary. Peak Flood Figure C10 shows that the Berth 201/202 works have the effect of reducing the peak flood flow speeds very slightly, predominantly over the deepened berth pocket, and the adjacent Berth 203 by up to around 0.02m/s. There is also a marginal flow speed reduction in the area of the Marchwood Moorings and Upper Swinging Ground, where flows are reduced by less than 0.01m/s on existing peak flows of less than 0.4m/s (i.e. representing a change from baseline of 2.5%). The area around the City Cruise Terminal (Berth 101) and in front of Mayflower Park also exhibit reductions in flood flow speeds of up to around 0.02m/s on existing peak flows of less than 0.4m/s (i.e. around 5% change from baseline conditions). Marginal increases in flow speeds occur (less than 0.02m/s) over small areas in the main navigation channel opposite Berth 203 and in the corner of the container terminal and Western Docks. This indicates that the deepening of the berth pocket ‘pulls’ the flow streamlines, marginally increasing flow speeds in the main navigation channel. A very slight localised increase in flow speeds (around 0.005m/s) is also observed in the main channel opposite Berth 101. There are no other notable changes in peak flood flows as a result of the berth works in the study area. The potential effect of these flow changes on flood tides would be to marginally increase sedimentation rates, principally in Berths 201/202 and 203, and in area of the Marchwood Moorings and Upper Swinging Ground and opposite the City Cruise Terminal, and potentially in the berth pocket of the Terminal. The actual modelled sedimentation patterns (Sediment Regime Modelling Section) show the net effect of spring/neap patterns whilst this section gives an interpretation of the flood/ebb tide effect alone. Peak Ebb As shown in Figure C11, existing (baseline) ebb flows are higher than flood flows and generally range between 0.6 and 0.8m/s at HW +4.3 hours at Dock Head on a spring tide. Comparison of Figure C11 with C10 shows a very similar pattern of change as for the flood tide. The reasons for the change are, therefore, effectively the same as discussed above but in the reverse direction. In general, the flow speeds vary (both increase and decrease) greater than on the flood tide but do not exceed ±0.02m/s. The main difference in overall distribution of the ebb flow speed changes compared to the flood tide are the marginal flow speed reductions (up to around 0.01m/s) over the shallow subtidal in front of Marchwood Military Port and Husband’s Yard.


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Time Series Analysis Although the previous discussion of effects of the berth works on hydrodynamics identified the locations in the study area where maximum change is likely, only two specific times on a single spring tide were considered. To determine how representative the previous spatial analysis is of all tidal ranges and times in the tide, time series analysis comparisons have been extracted from the models for the following three locations that are shown in Figure C12: Berth 206 at the container terminal (location Berth 206); In the main navigation channel, opposite Berth 201/202 (location Channel 8; and In the main channel, opposite Berth 102 (location Channel 10).

The time series plots for these locations within the estuary are shown in Figures C13 to C15. These plots show the relationship of the flow speed and direction changes against the water level changes for a set of tides with different tidal ranges. All three plots show that the changes to water levels are negligible for all ranges of tide (not discernable from background). The pattern of change to flows is similar at the three locations but differences occur in the magnitude of the change. In general, water flow changes increase with tidal range and decrease with distance from the berth works. Flow speed changes are shown to be greatest at the location in the main channel opposite Berths 201/202 (Figure C14), with maximum reductions in flows of 0.02m/s occurring during periods of low flows (representing of the order of 20% change from baseline flows during LW). At Berth 206, the changes (both increases and reductions) are on the whole less than ±0.005m/s, representing less than 0.5% of ebb flows and increasing to around 1% for peak flood flows (Figure C13). In the middle of the channel opposite Berth 102, Figure C15 shows that flows are again only very marginally changed (less than 0.005m/s). The negligible flow speed changes predicted will not have a measurable effect on the hydrodynamic working of the estuary. Sediment Regime Modelling Changes to the sediment transport patterns arising from the proposed Berth 201/202 works have been investigated for the Central Solent and Southampton Water using the Delft 2D local area model. This model has been calibrated for the SSC throughout the estuary, in conjunction with dredging records available since the previous capital dredge, as well as information based on differences in surveys, particularly in non-dredged areas. Whether increased sedimentation actually results will depend on a number of factors, including vessel movements, wave activity, sufficient sediment supply, and not least, the relative strength of the ebb flows to cause erosion. Since these factors will act to redistribute material deposited other than purely by tidal processes, the calibration also took account of the total annual average volume of material dredged from the estuary. Equally important, the modelled distribution of sediment was checked for areas where no accumulations have been recorded, as well as any known erosive areas.


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All modelling scenarios have been run through a complete spring/neap cycle, in order that the full effects of tidal range and water levels are represented within the model results. Two types of model outputs have been used for this analysis: Plan map plots of both SSC at different states of a typical spring tide and bed thickness change

at the end of the model simulation on a peak spring range tide. Plots have been produced at different scales in order to best represent the data and comprise the absolute levels of the parameter under the existing conditions and a difference plot from this baseline indicating the potential change as a result of the proposed deepening and widening of the approach channel.

Time series plots at the following locations to show how the sediment and erosion patterns vary

on the flood and ebb, on tides with different tidal ranges, and whether they are part of a rising or falling sequence (see Figure C12 for position of locations): - Berth 206 at the container terminal (location Berth 206); - In the main navigation channel, opposite Berth 201/202 (location Channel 8); and - In the main channel, opposite Berth 102 (location Channel 10).

Baseline Conditions Suspended Sediment Concentrations Figures C16 to C19 show the spring tide SSC and the change as a result of the berth works at the time of approximate peak flood flows, over HW, during peak ebb flows and around LW within Southampton Water. The baseline plots show that on a typical spring tide the SSC in the Solent is of the order of 70-80mg/l at all states of tide, being marginally lower at the time of peak flood flows in Southampton Water. At all states of the tide, depth averaged SSC reduce with distance up estuary and into the rivers. In general, the lowest values occur throughout the fastest flows of the ebb tide, with almost identical distributions during peak ebb flows and around LW (Figures C18 and C19). During this period, SSC are generally less than 20mg/l up estuary of Dock Head and in the River Itchen, reducing to around 10mg/l in the vicinity of the Container Berths. From Dock Head, SSC increase from about 20mg/l relatively uniformly to approaching 60mg/l around Calshot. On the flood tide, and over the HW Stand, SSC up to around 70mg/l penetrate to the vicinity of Fawley, before decreasing. Near Dock Head, SSC are around 30mg/l over HW compared to about 20mg/l during the time of peak flows. The time series plots in Figure C20 show how the baseline SSC vary between HW and LW, as well as for different ranges of tide at three locations along the estuary. All locations show the same pattern of variability both within a tide and over the different tidal ranges. In general, SSC increases on a rising tide (flood) to a peak at the beginning of the HW Stand and then falls during the ebb tide, with a minimum occurring about LW. At the end of the simulation, the difference in SSC between HW and LW on spring tides is of the order of about 7mg/l at all locations. This variation represents more than 25% of the peak spring tide SSC up estuary of Dock Head. This within-tide variability reduces to about 2mg/l or less over the smallest neap tides, representing about 10% of the absolute concentrations. The plots also show there is a lag of the order of a day in the maximum and minimum SSC in comparison to the equivalent magnitude of tidal range.


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On the smallest neap tides, mean depth-average concentrations of around 8mg/l occur in the time-series locations. On spring tides, the SSC reaches around 25 mg/l in the Western Docks and 18mg/l at Berth 206. This data serves to illustrate both the shorter and longer-term natural variability of sediment concentrations in this part of the estuary due to tidal forcing alone. This variability, however, has the potential to be increased further, but in a less regular manner due to natural wave disturbance and vessel movements. Accretion / Erosion Map plots, as used for the flow speed analysis, are presented in Figure C21. The plots show the distribution of annualised rate of sedimentation by scaling up the model results of a full spring/neap/spring tidal simulation over 15 days. The existing baseline distribution is shown in the top plot. The bottom plot shows the modelled change from the baseline conditions following the proposed berth works. Figure C21 shows the area to be net accretional over a year, for both the intertidal and subtidal areas. For the most part, however, annual rates of change are small, with large areas, particularly the shallow subtidal and intertidal areas accreting by less than 0.1m/year. It is important to bear in mind that the model is likely to overestimate the potential rates in all areas, since wave disturbance, from both natural and anthropogenic influences are not included in the model. The model, therefore, only reflects the distribution of sedimentation from tidally induced hydrodynamic and sedimentological effects. In the existing condition, dredging of the main channel is required to maintain navigation depths. The model gives a good representation of the sedimentation that occurs in the ‘quiet’ areas of the estuary, such as Ocean and Empress Docks, Marchwood Military Port, which all require regular dredging. Within the main channel, annual rates are lower, predominantly due to the higher ebb flows. Due to the non-representation of wave disturbance, the model baseline results will tend to overestimate the likely accumulation of sediment away from the navigation channel and underestimate the sedimentation within the channel. Taking account of the potential effects of wave disturbance, the annual changes for much of the area of the estuary will be small and well within the normal survey accuracy. The small changes that do occur are unlikely to be distinguishable in the field using standard measuring equipment. These plan plots only show the sedimentary distribution at a single point in time and alone give no indication on how the patterns develop through the spring/neap cycle, nor how permanent or transient they are. To establish these trends, time series plots have been extracted from the same three locations as for SSC (Figure C22). The effects of the berth works do not change the general sedimentary trends, just the magnitudes. Thus, to avoid repetition, these trends will be described along with the scheme-induced changes in the following sections. Effects of the Proposed Berth 201/202 Works Suspended Sediment Concentrations The effect of the proposed channel works on the SSC within the Central Solent and Southampton Water is shown at the bottom of Figures C16 to C19 at four states of a spring tide, as well as in time series at three locations along the estuary in Figure C20. In each case, the difference can be


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compared with existing SSC distributions within the estuary in order that the magnitude of the change can be set into context. Comparison of Figures C16 to C19 shows that, in general, the effect of the berth works is to very marginally reduce SSC (less than 0.1mg/l) in certain parts of the estuary at different states of the tide. These negligible reductions occur throughout the estuary between Redbridge and Netley at HW (Figure C17), and between Marchwood and Fawley, and to just beyond the Itchen Bridge, during peak ebb flows (Figure C18). No change is evident from background concentrations at other states of the tide, apart from a very localised negligible reduction (less than 0.1mg/l) to the immediate vicinity of the Berth 201/202 Works during peak flood flows (Figure C16). The slightly elevated levels of SSC (less than 0.1mg/l) on the Hythe to Fawley intertidal shore during LW are likely to be a result of modelling effects. The wetting and drying algorithm may exaggerate flow changes as model cells dry, which along with the shallow depths will over exaggerate the local flow speed and, therefore, potentially increase instantaneous erosion, increasing the depth-average SSC. In other words, this change in SSC, albeit negligible, is unlikely to occur in reality. The time series plots on Figure C20 show the magnitude of change and variability over different tidal ranges to be negligible, representing less than 1% change in SSC at each of the locations plotted. Accretion/Erosion The bottom plot in Figure C21 shows the predicted change in sedimentation within the Test Estuary as an annual rate (scaled from the results of the spring/neap/spring model simulation) and can be compared to existing baseline sedimentation rates shown in the top plot (although note the change in scales). No changes in potential sedimentation are predicted to occur downstream of Mayflower Park and upstream of the bottom corner of the container terminal. The only changes in potential sedimentation that are predicted to occur as a result of the proposed Berth 201/202 works are very localised and small in magnitude (± up to 0.015m/year). Figure C21 shows that the greatest increase in potential sedimentation rates will be within the deepened pocket of Berth 201/202 and at the City Cruise Terminal (Berth 101) by a maximum of 0.015m/year, which represents up to around a 25% and 10% increase from baseline at each location respectively. There will be a very localised and marginal increase in potential sedimentation (less than 0.005m/year) in the shallow subtidal area fronting Marchwood Military Port and Cracknore Hard. It is important to note that the effects of existing wave disturbance created both naturally and anthropogenically are not considered in these predictions, which are most likely to reduce the actual rate of settlement. The only areas were potential sedimentation will be reduced, albeit marginally (less than 0.01m/year), are in the main channel opposite Berth 203 and at the entrance and area fronting the King George V Dry Dock (Figure C21). To assess whether sediment accumulations are permanent or transient in nature, and how they build up during both different ranges of tide and times within a tide, time series have been extracted from the model to show the rate of change of bed thickness over a spring/neap cycle. These plots are shown in Figure C22. The time series are presented for the existing baseline conditions, post deepening (scheme) and the difference. The following analysis only considers the latter period of the time series, since it is considered that the earliest part of the sequence will still be affected by the initial model start up constraints.


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The sediment build up over a spring/neap cycle is evident both before and after the Berth 201/202 works at the main channel locations (Channel 8- opposite Berth 201/202 and Channel 10- opposite Berth 102). The precise locations are shown in Figure C12. The rates of sedimentation at these channel locations vary with the range of the tide and whether it is rising or falling within the normal spring/neap sequence. Generally, maximum rates of accumulation occur on falling ranges with absolute minimum rates occurring over the lowest ranges (neaps) and then intermediate rates as the tidal range increases. The plots also indicate that on spring tides, the rates of sedimentation reduce in the main channel locations and erosion occurs at the Berth 206 location. Following the Berth 201/202 works, the modelling indicates sedimentation rates in the channel directly opposite the works will be increased on all tidal ranges, but most notably on the larger spring ranges with negligible change over the neap half of the ranges (Figure C22). There will be no change to sedimentation rates following the berth works at the other two locations. As can be seen on Figure C21, location Channel 8 lies in an area of very slight potential sedimentation (around 0.001m/year) as a result of the scheme. The time series for this location (middle plot on Figure C22), shows that the net increase from the baseline over a spring/neap cycle is around 0.0002m (0.1mm). Sediment Dispersion Modelling During Dredging Introduction As a scoping requirement of the EIA, there is a need to determine the magnitude of sediment disturbance and the distributions of the sediments arising from the action of excavating the capital dredge sediments at Berth 201/202. The rate of disturbance (sediment discharge to the water column) is controlled by a number of factors, such as the method of dredging, which is primarily determined by the types of material that need to be dredged. Once the method is determined, the size of the plant depends on bathymetric conditions, such as depth of water and tidal range, as well as the time window of accessibility to the area. Once the material has been disturbed, the hydrodynamic characteristics of the area determine the rate and extent of distribution of the disturbed material within the marine environment, as well as indicating where the material may settle. The flow regime also determines whether any disturbed sediments that settle are subsequently re-eroded for further distribution over a longer timescale. Material Types A desk study has been undertaken of existing geophysical and geotechnical information in the berth pocket (Appendix B). From this, a site investigation was designed to supplement the existing data with the aim of developing a comprehensive understanding of the in situ properties of the different types of material to be dredged. In general terms, the materials to be dredged from the berth pocket are consolidated greensand and very stiff sandy clay of the Bracklesham Beds. These consolidated materials will require some form of mechanical dredging. Preliminary discussions with dredging contractors indicate that the dredge is likely to be undertaken by a backhoe dredger.


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Before the exact parameters for a realistic model simulation of the dredge process can be determined, there is a need to understand the phases in the dredge excavation cycle that give rise to disturbance. For the purposes of assessing the effects of dispersion during dredging, the model simulations employed the backhoe dredging method, effectively dredging at point (stationary) by mechanical means and loading to attendant barges. Hydrodynamic Conditions The hydrodynamic conditions in the area of Berth 201/202 are generally benign, with relatively low velocities occurring throughout the tide and being sheltered from wave activity. It should be noted, however, that the modelling considers only the distribution effects from natural processes on the sediments that have been disturbed by the dredge, but will not incorporate the effects of turning vessels and tug movements which already cause disturbance of alluvial type materials in this area. Dredging Down Time Whilst the dredging methodology will be chosen to minimise ‘down time’, inevitably some down time will occur (e.g. to accommodate passing vessels, mechanical breakdown etc.). This down time cannot be identified at this stage and cannot be incorporated into the modelling. The modelling will, therefore, represent a worst-case in terms of rate of disturbance of sediment by assuming a continuous dredging cycle. Dredging Method During the backhoe dredging operation, disturbance to the water column occurs during a number of phases of the ‘bucket cycle’: Lowering of the dredge bucket to the bed - during this phase, the release of sediment is likely

to be small, consisting of washing of sediments that may have remained adhered to the bucket from the previous cycle. This will occur throughout the water column;

The initial bucket impact will cause local disturbance to the bed. The actual scale will be a function of the bucket size, the impact speed, the nature of the bed material, as well as the immediate bed topography under the bucket;

The effect of the ‘ripping’ action of the bucket and the initial break out from the bed. Again this will be a near bed effect, probably restricted to around the bottom 2m of the water column;

Wash off of the exposed surface of the material in the bucket as it is raised through the water column. The amount released will again be a function of the speed of the flowing water, the nature and particle size of the material in the bucket and rate of raising the bucket. This release is likely to occur throughout the water column. Additional material could be released from the bucket as it breaks the water surface and slewed over the barge; and

Finally additional material may be released from the barge if overflowing is required to optimise the load. The need will depend on the number of barges available and distance of travel to the deposit, along with the character of the material within the barge. This release of the sediment could be onto the surface or at the draught of the barge depending on its design. For the Berth 201/202 dredging, the modelling has assumed that overflowing will not be required.


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Model Scenarios The previous sections have identified the considerations that have needed to be taken into account in order to provide a comprehensive assessment of the dispersion effects of disturbed materials arising from the dredging process. There are many highly variable facets to the dredge disturbance process, and it can, therefore, be difficult to determine the exact rates of disturbance. The modelling scenarios are a best representation of the characteristics of the dredge method since it is not possible to model all the phases of the dredge process. Equally, accurate discharge rates are difficult, if not impossible to measure, and, therefore, only general rates can be used. However, assumptions that have been adopted cover the worst-case scenarios. The modelling provides the means to quantify the extent of distribution from the source and the rate of accumulation over the entire time of the dredge to allow an assessment of the impact on sensitive areas, for example, the potential for smothering of benthic and shellfish areas. The following sections give details on the determination of the modelling disturbance rates and the method of simulation employed. Disturbance Rates The rate of disturbance around a dredger will depend on the variability in material, types of equipment (its age, quality and level of maintenance) and human operator characteristics. One of the most comprehensively designed studies took place in the Netherlands during the late 1980s and early 1990s (Pennekamp and Quaak, 1990 and Pennekamp et al 1996). Water sampling and turbidity measurements were made in a systematic grid around a working dredger measuring the plume generated over a few hours of dredging. The results were presented as the ‘S’ parameter, which was a measure of the mass of dry sediment released per cubic metre of in situ material dredged. This and other data were reviewed by Kirby and Land (1991) who developed indicative ‘S’ factors for a variety of dredging plant working in soft marine and harbour muds. Burt et al (2007) reported detailed monitoring around a small grab dredger and found an average ‘S’ factor in accord with those indicated by Kirby and Land, although short term values ranged from about half to twice the average rate. This generally indicates that use of the indicative ‘S’ factors with careful consideration of the relative environments (to those where the ‘S’ factor was derived) and material types being dredged can be used to give a good approximation of the average release rates for modelling purposes. To use the data presented by Kirby and Land (1991) to derive modelling inputs, there is a need to know the approximate production rate, size and sequence of working of the individual dredger. The details for the derivation of the disturbance rates used in the modelling are set in Annex C2 and described in general terms in the following section. Simulation Method It has been assumed that the dredging of Berth 201/202 will be undertaken using a large backhoe dredger, effectively continuously loading a ‘stream of barges’ over the period of simulation.


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The materials to be dredged in the berth pocket are the consolidated greensand and very stiff sandy clay of the Bracklesham Beds, with possibly some areas covered by a thin layer of recently deposited soft mud. The greensand, whilst very dense in situ, is known to easily break down to the constituent parts once dredged and, therefore, has the potential to increase SSC/turbidity levels locally and disperse widely. The modelling was undertaken for the following four scenarios: 1. Dredging a barge load over the HW stand on a neap tide. The detailed model output should

represent the case for maximum local SSC with minimum dispersion; 2. Dredging a barge load during the flood on a neap tide. This gives an indication of the

maximum extent of up-estuary dispersion from the location; 3. Dredging a barge load during the ebb on a neap tide to determine the initial down-estuary

plume effects and give a history of initial plume decay; and 4. Dredging continuously over a spring/neap cycle to determine the longer-term effects of the

disturbance, taking account of any secondary redistribution of settled sediments. The outputs from this scenario should indicate the overall extent and magnitude of deposited sediments resulting from the dredging works in the upper estuary.

For these scenarios, the rate, times and locations of disturbance from the backhoe dredging process are given in Annex C2. Results of Modelling Dredge Dispersion Scenarios This section presents the results of the modelling scenarios described in the previous section. Map plots showing the maximum change in SSC and bed thickness resulting over a 15-day spring/neap cycle taking account of loading times, barge size, material type and timing of barge movements for an average case are presented in Figure C23. These plots give a measure of the worst-case at any model cell, without due consideration of length of time it occurs, when it occurs, or modelling simplification etc. The plots, therefore, identify areas and specific locations where in-depth analysis is required with additional plots at various times in the modelling sequence or specific time-series analysis. These additional analyses are presented in Figures C24-C27 for different times and locations in order to describe the resulting distributions. Suspended Sediment Concentrations The top plot of Figure C23 shows that additional SSC for nearly all subtidal areas of the estuary are not expected to exceed 10mg/l above background, with the exception of the channel opposite the middle of the Western Docks, where peak increases of the order of 30mg/l can be expected. In these areas, existing background concentrations do not exceed 30mg/l and average considerably less. The detailed analysis of time series in Figures C24-C27, indicate that maximum values in SSC created by dredging could be about 3 times the average effect. The average change from the backhoe dredging would therefore be in the region of 3-10mg/l in the main channel i.e. representing in the order of 20-60% above background concentrations during the period of the Berth 201/202 dredge and probably for around 15 days following its completion.


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Considerably higher maximum concentrations are predicted (circa up 150mg/l) on the intertidal and in the creeks of the Bury Marshes. The model also indicates increased concentrations of 30-100mg/l over the intertidal between Cracknore Hard and the Marchwood Yacht Club and all along the Dibden Foreshore. High SSCs are also shown to result in the lee of Hythe Marina (up to 200mg/l) over the lower and middle intertidal around the Fawley Reclamation (circa 100mg/l). On the east side, the concentrations are generally enhanced less, with the highest adjacent to the foreshore. When interpreting this, it should be considered that the SSC results are depth-averaged. In shallow areas, even a small mass of material will create a high concentration. It is, therefore, likely that modelled peak concentrations over the intertidal will be very short period ‘spikes’ as material is deposited. It is important to note that the dredge simulation represents the finest material likely to be disturbed and assumes the material dispersed will act as particles with a characteristic size of 10µm. Overall, this scenario, assuming the release rate is correct, is likely to over estimate the SSC enhancement, and distribute the sediment wider than might actually occur. This will in turn, tend to slightly underestimate the amount of bed accumulation. Accretion/ Erosion The bottom plot on Figure C23 indicates where the disturbed sediment is likely to settle. These bed thickness levels are calculated for an average in situ density of about 1100 kg/m3. However, over the period of the dredge, consolidation will occur in the undisturbed areas, reducing the maximum bed thickness represented in the plot. Figure C23 shows that most settlement will be confined to the area up estuary of Dock Head. The maximum accumulations on the bed predicted in the upper Port areas are likely to remain, as the amount of re-erosion in the upper estuary is considerably less than further down estuary. Small accumulations (less than 5mm) occur at a few ‘quiet’ (natural sink) locations down-estuary e.g. Ashlett Creek. The vast majority of the material will re-deposit within the deepened berth pocket of Berth 201/202 and will need to be re-dredged. Other notable accretions (up to around 60mm) will occur in the slack water (eddy areas) in the Upper Swinging Ground and at the Cruise Terminal Berths 101/102 and the end of Mayflower Park. Outside the main channel, accumulation of up to 30mm will result in the Marchwood Berths over a spring/neap cycle. A similar accumulation will occur in the pocket of Marchwood Wharf, with 2-3mm of accumulation over the adjacent intertidal. The largest accumulations outside the dredge channel in the area of the container terminal will be in the Bury Swinging Ground, although the depth change will be small amounting to about 10mm. A small amount of accretion is also evident in the Eling Channel (2-3mm) over a spring/neap cycle. Sedimentation will occur in the outer half of Ocean Dock and the main berths around Dock Head in the River Itchen (up to 20mm). Approximately 182,000m3 of capital dredging is required from Berth 201/202, much of which is consolidated stiff clay and sand. It is proposed that all this material will be dredged by the backhoe loading barges. The modelled scenario represents the removal of about 262,000m3 of in situ material over a period of 15 days and, therefore, the sediment dispersion simulation appertains to around 1.4x the total dredging commitment. This suggests that the bed thickness changes indicated by the modelling are likely to be around 30% lower in reality. Based on this multiplication factor, of the order of around 40mm will actually accumulate in the Berths 201/202 and corner of the Upper Swinging


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Ground as a result of total removal of material from the berth pocket. It should also be noted that accounting for down time, the overall period of the dredge is likely to last approximately four weeks. The modelling, therefore, overestimates the rate of input of sediment into the estuary. Time Series Analysis

To investigate the distribution plots shown in Figure C23 further, a set of time series through the complete length of the simulation are shown for a number of locations in Figures C24-C27 with the specific sites indicated on Figure C12. This diagram shows more locations than have been presented here. Only those which illustrate the main trends and magnitudes of change are shown. Main Navigation Channel Location Channel 08 (Figure C24) illustrates the effects of the dredging for a location within the main channel adjacent to Berths 201/202, at 10 minute intervals throughout 15 days of dredging followed by a further six days when no dredging was taking place. With respect to additional SSC, peak increases were generally up to about 8mg/l on both spring and neap tides whilst dredging of the berth pocket was taking place. Such levels were only recorded for short periods of time (a few hours), and generally falling to around 4-6mg/l for the majority of the time during the period of dredging. The 'expanded' sections for the spring and neap tides show that, in general, suspended sediment concentrations are highest during the ebbing phase of the tides. During the dredge, accretion will occur on the bed during HW (up to about 7mm over the 21 day period of the simulation). The proposed dredging of the berth is anticipated to last approximately four weeks and, therefore, the source of this sedimentation will cease following this period. This location in the channel is already maintenance dredged so this sedimentation, albeit marginal, will potentially add to this process. There is also likely to be some redistribution of material by waves, reducing these sedimentation levels and transporting them to the ‘quieter’ areas of the estuary e.g. berths. Location Channel 10 (Figure C26) shows the effects of the dredging slightly further downstream of Berth 201/202. Here, the maximum concentrations reached will occur for very short instances and actually increase following the cessation of dredging activities, reaching a peak of around 20mg/l for less than an hour. These increases are largely due to the re-suspension of recently deposited material. They generally occur on the ebbing phase of certain tides when there is sufficient energy to resuspend material, thus, locally increasing suspended sediment concentrations in the water column. Sedimentation levels will accumulate gradually over the dredging period reaching magnitudes of 2.5mm. This will reduce following cessation of the dredge to background and for intermittent periods around 1mm for a few days, before being completely eroded and returning to existing levels. Berth 206 Location Berth 206 (Figure C25) illustrates the dispersion effects of dredge disturbed material up estuary of the Berth 201/202. At this location there will be elevated suspended sediments of up to around 6mg/l (Figure C25). These peaks, occurring again around the ebb phases of the tide, last for very short periods of time, reducing to around 0.5mg/l above background. Elevated SSC of the order of 6mg/l will still occur during the next spring/neap cycle following the cessation of dredging.


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Some (negligible) accretion occurs (up to 0.75mm) as tidal range increases, however, this is re-eroded down to around 0.5mm on the highest range spring tides and subsequently decreasing range tides. Following cessation of the dredging, the amount of sedimentation will be negligible (less than 0.25mm). These small accumulations will merge into the background variability within a spring/neap cycle after the end of the dredging. Marchwood At the Marchwood Moorings Location (Figure C27) there will only be very marginal increases in suspended sediments over the period of dredging of up to around 5mg/l and generally falling below these levels. Following the cessation of dredging, after the approximate four-week dredging programme for the Berth 201/202 works, there will generally be continued levels of suspended sediment concentrations of a similar order as during dredging. Potentially greater momentary peaks will occur as material that has accumulated in nearby intertidal areas is re-eroded when greater range tides and stronger flows can reach and remove the recently deposited material. Up to around 1-2mm of material will accumulate at this location over the period of dredging. Following cessation of the dredging activities, a negligible amount of material will remain accumulated for short periods of time over subsequent tides (up to around 0.025mm). Sediment Dispersion Modelling During Disposal Operations Model Scenarios As a scoping requirement of the EIA, there is a need to determine the magnitude of sediment disturbance and the distributions of the sediments arising from the deposit of the sediments at any licensed deposit/beneficial use location. The excavated material from Berth 201/202 will be deposited at the Nab Deposit Ground by direct release from the bottom of the hull of barge, with varying volumes depending on the size of barge and the material type being dredged. The frequency will be variable depending on the number of vessels used. Individual deposits will be quite disparate in time due to the generally long haul distance to the deposit ground. The modelling results show that a load is relatively quickly dispersed when in particulate form. The model scenario used to evaluate the dispersion characteristics that will arise from deposits of the fine material dredged from Berths 201/202 at the Nab Deposit Ground has assumed that all material will break into its constituent particle sizes. This will tend to maximise the extent of dispersion and SSC away from the deposit ground but will underestimate the initial amount of deposition. For all modelling, it has been assumed that the deposits from the backhoe loaded barge will take a period of 10 minutes to discharge. The total number of deposits over a spring/neap cycle has been distributed uniformly over the area of the deposit ground by dividing the area into sectors and depositing each load in a different sector of the deposit ground. This practice will ensure the modelling represents the full extent of dispersion from the deposit ground, without creating the potential for large depth changes at any single location should the material be retained. Each model run, with input of sediment, starts and finishes at peak spring tide ranges. The model was then run for a further 7 days to determine any continuing change in SSC or deposition/erosion following


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cessation of the disposal activities. The release rates and sediment characteristics for the disposal modelling scenario are provided in Annex C3. Once the material is released from the backhoe barge, much will descend quickly to the bed, whilst a proportion will immediately advect with the ambient flow. In the modelling, an assessment of the proportion of material that will descend to the bed has been made, based on the particle size of the material likely to be in the barge. This material for modelling has been placed as a source in the bottom 2m of the water column with the remaining material input between the draught of the barge and the bed. Plots of the maximum extent and magnitude of dispersion from the deposit ground for SSC and bed thickness change are presented, both for the individual particle sizes modelled and the combined effect for the material deposited over the spring/neap cycle (Figures C28 and C29). These plots represent the effect of the complete dredge volume. Since the maxima plots do not give any idea of the transient nature of the dispersion process, time series plots of SSC and bed thickness change are presented in Figures C30-C33 for certain locations. Results of Modelling Deposit Scenario Plots of the maximum SSC and extent of effect arising from the deposit scenario required to dispose the material arising from the Berth 201/202 works are shown in Figures C28. The combined plot shows the extent of dispersal of sediment of all particle sizes from the Nab Deposit Ground, which covers an approximately rectangular area of around 920km2 (46x20km), from the east coast of the Isle of Wight eastwards and extending from Bracklesham Bay in a SW direction. In addition, sediment is distributed through the east and west Solent and a small amount moves into Southampton Water and Portsmouth Harbour. All plots indicate the dominant axis of the initial plume away from the disposal area is on a WSW to ENE axis with the majority (ratio 2.5:1) of material initially moving to the WSW, before being dispersed over a wider area over time. Maximum suspended sediment concentrations near to the point of disposal will reach up to around 200mg/l (Figure C29), but in general will be below 25mg/l (Figure C30), reducing to below around 20mg/l within about 2.6km down flow (Figure C31). The maximum width of the plumes with concentrations in excess of 100mg/l is of the order of 250m. Maximum accumulation at the bed arising from the disposal of the material at the Nab Deposit Ground over a spring-neap cycle will be negligible (below 1mm) apart from a few isolated spots within the Deposit Ground, in which maximum sedimentation will be of the order of 2mm. To give another representation on how the initial plume decays with distance from the disposal site, transects of the depth average SSC and bed thickness accumulation for the different sediment sizes dredged from the Nab Channel are presented for different times throughout the model simulation in Figure C34 and C35. The lines of the profiles run both along and across the plume, as shown in Figure C29. After a single day of disposal operations the sediment has spread approximately 2km NE and 15km SW, with the bulk of the material about 2km SW of the deposit ground, where accumulated depths at the bed are negligible (less than 0.1mm). This distribution, after one day is related to the exact location


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of the initial deposits (at the southern end of the deposit area) and the state of tide at which the concentration is extracted from the model (peak ebb flows). As time progresses and the tidal range reduces from springs to neaps, the change in bed thickness in the disposal area increases but is still negligible (around 0.2mm), while the SSC continues to be around 25mg/l. At this time (7 days after the start of disposal operations) changes in bed thickness extend 7km to the NE and 10km to the SW of the disposal ground. After 14 days, the axis of the main SSC plume has moved inshore towards Sandown Bay. In the scenario depicted in Figures C34 and C35, deposits stop after 9 days, therefore, the times of 14 and 21 days reflect the migration of the deposited sediment firstly on spring tides and then neap tides a week later. These plots show the migration of sediment mass continues both inshore and southwards with concentrations falling to around 10mg/l (albeit over a wider area) on springs and then to below around 5mg/l on neaps a week later. The plots tend to indicate that accumulations on the bed will be negligible (less than 0.2mm) at any time. After 14 days (i.e. 5 days following the completion of disposal) nearly all of the deposited material will have dispersed from the deposit ground. Time series locations, Sites 9 to 11, are most representative of the initial disposal plume conditions, whereas the remainder of the sites show how the material disperses more widely with time. Site 11 is for a location at the centre of the deposit ground, but not specifically at an exact disposal location (see Figure C29 for location). Figures C30 and C31 show that during larger range tides the deposited material at Sites 9 and 11 is more quickly dispersed, thus creating faster dilution with peak SSC reaching a maximum of 70mg/l within the disposal ground and averaging less than 10mg/l. As the tidal range falls, the flow speeds disperse less material immediately and concentrations increase with peaks of up to 150mg/l and tidal averages of around 15mg/l. As soon as disposal ceases, SSC return to background levels as the material is moved away from the specific location (Figure C30). The sedimentation plots indicate maximum depths of accumulation of sediment of up to around 0.4mm, occurring for a period of about 2 hours over HW and LW periods on neap tides. All material is then re-eroded on the following flood or ebb peak flows. Further afield, time series from Sites 4 and 5 (Figures C32 and C33), indicate how the centre of mass of the sediment migrates towards the eastern shores of the Isle of Wight. At these locations the effect of the disposal practice is not evident for about 4 days (starting on springs). If the initial disposal occurred on neaps it is very likely that effects inshore would not be evident for a few more days following the first deposit. Again SSC are increased on spring tides, with peaks of around 170mg/l for short periods at Site 4 and around 130mg/l at Site 5 (closest inshore), but with a tidal average of around 20mg/l. The maximum accumulation thicknesses would be around 0.5mm and would only be on the bed for short periods around HW and LW. The dispersion pattern that is predicted indicates that the deposit of the capital dredge sediments from Berths 201/202 will only have very transient and temporary effects on the character of the bed (maximum accumulations of around 0.5mm), with all effects of the disposal returning to background conditions within a few days to around a week of cessation of the disposal operations. The analysis of the enhancement of SSC has been undertaken on a depth-average basis. However, in reality the concentrations will vary throughout the water column. Figure C36 shows that for the dredged material from Berth 201/202, most of the material is likely to be moving around the Solent within the bottom 2m (i.e. as near bed load). Within this layer, peak concentrations over neap tides will be of the


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order of 100mg/l over the deposit ground whilst the depth-average concentration is generally an order of magnitude lower, with very little (by comparison) in the water column above the depth of the loaded draught of the barge. The amount carried near the bed tends to reduce inshore as depths become less. At Site 5 in the shallowest water and closest inshore, the depth-average and near bed concentrations are more similar. This information suggests that for much of the area where sediment is dispersed, turbidity levels affecting light penetration will be relatively low throughout most of the water column (i.e. relatively clear). Future Maintenance Dredging With the existing Berth 201/202 depths, maintenance dredging is undertaken every few years at irregular frequencies. The average volume of material removed in each maintenance dredge campaign equates to around 1800m3 sedimentation per year. The assessment on the effects of the proposed works on the berth maintenance dredge commitment has been made using the results from the modelling of various parameters, such as the effect on flow patterns and the sedimentation patterns. The specific modelling assumptions, method and accuracy of calibration has been considered, along with the potential effects of processes not included in the modelling, primarily episodic natural and anthropogenic disturbance effects (e.g. waves). In the sediment calibration, the primary aim was to reproduce the distribution of observed sedimentation within the areas dredged and the relative thickness of sedimentation, whilst keeping the suspended sediment concentrations of the correct order throughout the estuary. It is recognised that this process has the potential to overestimate the amount of accumulation occurring in non-dredged areas, with re-erosion and redistribution of material taking place in reality and, therefore, some of the material adding to that directly depositing in the maintained areas. The future maintenance dredging commitment has been estimated by calculating the volumes of material accumulated over a year (i.e. predicted rates of sedimentation over the relative area of the berth pocket) and summing it to existing maintenance requirements (See Sediment Regime Modelling Section). Figure C21 shows that following the deepening of the berth, the potential for sedimentation is increased marginally, with accumulations of around 0.01m per year predicted across the berth pocket area. The plots indicate changes predicted outside of Berth 201/202 are so negligible that they would not be perceptible when the natural variability of the system is taken into account and, therefore, little or no additional dredging is predicted elsewhere. The maintenance dredge analysis indicates that the annual maintenance dredge commitment for Berths 201/202 will increase by the order of 400m3/year. Therefore, the total volume accumulating at Berths 201/202 will be around 2200m3/year. This represents an increase of about 20% with respect to the average volume that is currently dredged from the berth. This change is considerably smaller than the annual variability in the existing maintenance dredging requirements (± 2500m3/year). Assuming the increased sedimentation is removed from Berths 201/202 by the TSHD (with hopper capacity of 3900m3), the number of dredger loads per year will be increased by 0.1 loads for the maintenance of the berth, which equates to an increase of 1 dredger load over ten years. Therefore, overall, little noticeable change to the existing maintenance dredge practice will occur. The pocket, however, will be a 'sink,' therefore, any sediment disturbed by shipping is likely to accumulate in the pocket. This effect


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is not simulated by the model, so the maintenance commitment for the pocket may be greater than that indicated by the modelling, however, other areas may require less dredging. In-Combination Modelling Chapter 18 of the EIA sets out the strategy for the assessment of cumulative and in-combination impacts of other plans and projects under consideration within the area of influence of the Berth 201/202. Where details of the current plans are available, either from engineering drawings or schematic plans a best estimate of the likely geometric detail that might occur has been represented within the local area 2D model. Some of the schemes have little detail and are close to the HW mark, making representation in the model relatively simplistic. The larger developments involving dredging, structures or significant reclamations have all been implemented in some form, making realistic dimension assumptions based on the descriptions where no specific details are available. The main developments included in the modelling analysis were: ABP Southampton Approach Channel Dredge; Layout 5 arrangement for the proposed rubble mound breakwater, secondary approach

channel dredge and marina at Cowes; The proposed marina at the location of Husband's shipyard (based on Oceanic Estates Ltd

drawing 572/M08-03); The proposed re-development at Woolston. The majority of the change adjacent to the HW

mark was not resolvable in the model, so most effects were limited to simulation of increased drag from the proposed pontoon system; and

The potential reclamation and dredging at Town Quay and Mayflower Park,. Details of other proposals within Southampton Water were insufficient to design a meaningful representation in the model. Modelling has not been undertaken of development plans outside of the Central Solent area covered by the local area 2D model since the modelling of the approach deepening alone created little effect in these areas. The Lymington Breakwater, Seagrove Bay Coastal Protection scheme and the Portsmouth Harbour dredge have, therefore, not been modelled in this in-combination analysis. All developments as described above have been implemented in the model and the effects in-combination with Berth 201/202 have been presented as map plot differences from the Southampton Approach Channel Dredge alone at specific times in the tide. Time series have also been extracted from the vicinity of the developments to investigate the potential effects through all tidal states. These have not been specifically presented but were reviewed to inform the following analysis of the in-combination effects. Water Levels Figure C37 show the modelled difference in the absolute HW and LW levels for the largest spring tides as a result of the inclusion of the additional developments. The calculation is the in-combination scenario minus the approach dredging alone, therefore, a negative means the additional developments will cause a lowering of water levels compared to the effect of the deepening alone. These plots are equivalent to those shown on Figure C38 for the Southampton Approach Channel Dredge alone.


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At HW, Figure C37 shows that in general water levels will predominantly be unchanged or marginally reduced (for the most part less than 1mm) in all areas down estuary of Dock Head and within the Rivers Itchen and Hamble. Some small areas of marginally increased levels (circa 1-2mm) are shown over the intertidal areas on the west side Southampton Water and around Warsash. Increases in water level of up to 1cm are shown in the Upper Hamble, however, these cannot be considered to be accurate due to the resolution of the modelling in this area. Up estuary of Dock Head, particularly Mayflower Park a distinct change to the general pattern, down estuary occurs. Here the highest water levels are raised all by an average of around 3mm, although immediately adjacent to the new development at Mayflower Park the magnitude of change approaches 1cm. This distribution of change is very consistent with the locations of the additional developments, most of which are in this section of the estuary. Comparison with Figure C38 shows that, in general, down estuary of Dock Head the additional developments tend to marginally enhance the effect of the deepening alone and cause more change within the Central Solent. The magnitude of this change, except in close proximity to the Cowes Breakwater development, is less than 0.5mm and is not consistent over the whole area. At LW a similar distribution of water level change is seen (Figure C131) to that at HW, with marginal reductions in water level, but generally 0-0.5mm, below Dock Head and into the Solent. Changes over the intertidal are a residual of the modelling process since at these locations at LW the areas will be dry. Considerably larger reductions (exceeding 1cm) are shown around the Cowes Breakwater development and extending in to the Medina. This is consistent with the detailed modelling of this development (ABPmer, 2008). With respect to the Cowes Development the modelling of water levels tends to indicate that interaction of changes as a result of the Southampton Approach Channel Dredge and the Cowes development is minimal. Similar to the effects at HW, the change to water levels up estuary of Mayflower Park with the additional developments is an order of magnitude greater than for the rest of the estuary. Throughout the Western Docks and container terminal area LW levels are reduced in general consistently up to about 1cm. An anomaly is a localised increase in water level in a narrow strip at the edge of the Bury Swinging Ground. Comparison with the LW change resulting from the approach dredging alone (Figure C38) shows that of the order of 50-60% of the increase in LW level resulting from the approach dredging would be reduced when the combination of the developments above Dock Head take place. These changes will have implications with respect to the longer-term change in intertidal habitat within the estuary, particularly about Mayflower Park. Should all the developments take place to the same scale as modelled, the predicted loss of intertidal within Southampton Water from the channel deepening alone will be reduced. Down estuary of Dock Head the change would not be measurable, with the reduction in loss for the most part confined to the River Test up estuary of Dock Head. A specific calculation of the reduction, however, would not be appropriate as this effect could not be guaranteed as the individual developments may not occur, and if they did, the original proposals could change considerably.


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The modelling of water level change does, however, tend to indicate that there will be cumulative and in-combination effects due to the interaction of the effects on the system processes from each development. The analysis does not in its own right identify the likely contribution from each development. Flow Speeds Figure C39 shows the change in approximate mean spring flows between the deepened approach scenario and the in-combination scenario throughout the model area, in a similar manner to the water level changes. The top plot in Figure C39 presents the distribution of change attributable to a time 1.5 hours before the first HW for the tide measured at Dock Head (i.e. approximately peak flood flows within Southampton Water). The effect at the time of approximate peak ebb flows (HW +3.3 hours at Dock Head) is shown in the lower plot in Figure C39. These plots clearly show that the most significant effects on the flow patterns within the estuary result from the reclamation and dredging works associated with the most recent architect plans for the Town Quay to Mayflower Park Development. Separate effects are shown for the Cowes Breakwater and they clearly show there is no interaction between these two areas. Also with respect to flow speed changes the further developments have no additional effect on the peak flow dynamics of the estuary below Dock Head or the channel areas of the Central Solent. The changes shown at Cowes have a similar extent and general pattern of change as for the specific detailed modelling of that development, which should be referred to quantify any change, since a specifically calibrated model with high resolution has been used to determine the impacts of this development alone (ABPmer, 2008). The present modelling indicates no interaction with the Southampton Approach Channel Dredge therefore this development is not considered further in this report. Figure C40 shows an enlargement of the area up estuary of Dock Head so the precise nature of the flow speed changes can be assessed in this area. From this diagram it is clear that the most significant effect is caused by the developments at Mayflower Park, with a small interaction with the marina at the Husband's Yard. By comparison the effects of the Berth 201/202 redevelopment would appear to be minimal contribution to the in-combination assessment. Flood Flows Off Mayflower Park, the peak flood flows are diverted towards Dibden Bay, predominantly by the Mayflower Park reclamation. The cross section area of the estuary is reduced, as is the accommodation volume of the estuary. This has the effect of locally increasing the peak flow speeds over those which were attributable to the approach deepening alone, which are shown for this area of the estuary on Figure C41, which were reductions of up to 0.02m/s. This change would return the peak channel flows back to approximately those that exist at present (i.e. the baseline condition). Flows over the shallow subtidal and intertidal to the west will be increased by about 0.02 m/s (circa 10%) compared to the baseline. In these areas the approach deepening alone had little effect. In general these increased flows tend to remain to the west side of the estuary and decay towards the container terminal.


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Immediately in the up stream lee of Mayflower Park (where the return to the quay of the Western Dock has been increased) flow speeds are reduced by of the order of 0.1 m/s in the area of Berth 101 reducing to no significant change at Berth 109. This represents a maximum reduction of the order of 50% in areas where flows rarely exceed 0.2 m/s, therefore additional sedimentation is likely to occur on the flood tide within the berths of the Western Dock, with the volume reducing with increasing distance from Mayflower Park. The flood tide plot also shows that the main channel flows from around Berth 105 up estuary are not predicted to be significantly changed by the additional developments, although flows will be reduced by up to 0.02m/s in the new Berth 201/202 pocket as well as the existing container berths. Ebb Tide On the ebb tide (Figure C40) the same diversion in flow by the reclamation works is evident, but flow speeds are increased by a larger amount than on the flood (up to about 0.08m/s circa 20%). This increase is as much as double the magnitude of decreases that is predicted to occur in this area for the deepening alone. The plot shows the most significant effects are retained within the main channel and near to its edge, down stream. The flow directions on Figure C42 show this is as a result of a predominant flow direction to the west side, which is at an angle to the mudflat/subtidal orientation, thus moving flows towards the channel. This and the effect of the reclamation, therefore, cause a constriction in the flowlines towards the outer edge of the channel. In the lee of Town Quay and through the marina, ebb flows will be reduced considerably with effects extending beyond Dock Head. Over the Marchwood foreshore, the combined effects of all developments is to increase flows in general (compared to the Southampton Approach Channel Dredge alone), except where dredging has occurred for the Husband’s development. These changes partially offset the flow reductions in this area that occurred due to the approach deepening alone. The net effect would therefore be little overall change over the intertidal areas to the existing baseline condition, should all developments take place. Within the area of the Berth 201/202 redevelopment, little consistent change in the flow patterns is evident from the in-combination modelling. Sediments Suspended Sediment Concentrations The change resulting from the inclusion of the additional developments has been examined at the time of peak flood and ebb flows, HW and LW within Southampton Water. The changes were very small, therefore the plots have not been presented but showed that at some time throughout the tide suspended sediment concentrations would be marginally reduced (generally less than 0.2mg/l), i.e. of the order of a further 10% of that which occurred from the approach deepening alone. The Cowes Breakwater caused effects, which were confined to the breakwater location, where a reduction of the order of 1mg/l was predicted with a marginal increase for the most part less than 0.5mg/l within the estuary. Again there is no evidence of interactive effects between the approach dredging and the Cowes Development.


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Sedimentation Figure C43 shows the overall effect of the modelling of the in-combination scenario as the difference in annual sedimentation rate compared to the predicted sediment distribution for the Southampton Approach Channel Dredge alone. This plot shows that despite the small (negligible) reductions in SSC over the whole estuary the changes in the distribution of sedimentation were predicted to be restricted to up estuary of Hythe and confined to the area of the Cowes Breakwater and the Medina Estuary. The detailed effects of the Cowes Breakwater are reported in ABPmer (2008). The present modelling shows there is no in-combination effects with other planned developments. Figure C44 shows an enlarged view of the change in annual sedimentation from Hythe up estuary, which shows a similar overall pattern of change to the net effect of the flow change distribution shown in Figure C40. In general, this shows that the additional developments, predominantly the Town Quay to Mayflower Park reclamation, will increase sedimentation by up to 0.05m/year for about 500m up estuary of the northern end of Mayflower Park and a similar distance down estuary of Town Quay. The potential for an increase in maintenance dredging would therefore arise, particularly in Berth 101/102 and the immediate channel approach. In addition, maintenance dredging is likely to be required across the entrance to Town Quay marina, with a slightly lower rate within the marina itself. This represents an increase in sedimentation approaching an order of magnitude greater in these areas than predicted to arise from the Southampton Approach Channel Dredge alone. Similar rates of accretion are also predicted to occur within the deepened areas of Husband’s proposed marina. Potential sedimentation rates, however, over the main channel, shallow subtidal and intertidal between Dock Head and the start of the container terminal are predicted to reduce between 0-0.02m/year. With the Southampton Approach Channel Dredge alone, little change was evident over much of this area. Monitoring over the last 10 years has shown that net erosion has been occurring, therefore it is possible that whilst the channel dredge alone potentially improves the situation, the additional developments could increase intertidal erosion, particularly along Dibden Bay. The overall modelling of water levels, flows and sediments suggests that the vast majority of this effect is due to the Mayflower Park development. In comparison, the effects of Berth 201/202 and the Husband’s development are small and remain local to the footprint of the development. In the case of Berth 201/202, the main sedimentary effect is an accretion within the pocket of between 0.01-0.02m/year giving an additional maintenance dredge commitment of less than 1000m3 per annum. The pocket, however, will be a 'sink,' therefore, any sediment disturbed by shipping is likely to accumulate in the pocket. This effect is not simulated on the model, so the maintenance commitment for the pocket will be greater than that indicated by the modelling, however, other areas may require less dredging. Summary Assessment of the in-combination effects of other projects and plans indicates that the only significant interactive effects that would change the assessment of the Berth 201/202 Works alone are those that occur up estuary of Dock Head, namely the reclamation (predominantly) and berth dredging between Town Quay and Mayflower Park, the Husband’s marina development and the ABP Southampton Approach Channel Dredge. The Cowes Breakwater has its own effects, but the modelling indicates this


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will not be significantly affected by the Southampton Approach Channel Dredge and vice versa, with all effects remaining in close proximity to the works and the Medina Estuary. Nearly all in-combination effects within Southampton Water are restricted to the River Test above Hythe, with the vast majority of the change resulting from the Town Quay to Mayflower Park development. The most significant effects of the in-combination modelling with respect to the Southampton Approach Channel Dredge alone are: HW levels up estuary of Mayflower Park will be increased on average by 3mm, but up to 1cm

immediately adjacent to Mayflower Park. Down estuary of Dock Head the levels will be predominantly unchanged or marginally reduced thus minutely enhancing the effect of the deepening alone;

LW levels down estuary of Mayflower Park are lowered a small amount (0-0.5mm). Within the Western Docks and container terminal the level is reduced consistently by about 1cm (i.e. reducing the increase that occurred from the approach deepening alone, by about 50%);

Reductions in LW levels of the order of 1cm are caused by the Cowes Breakwater, which do not appear to interact with the approach deepening;

The most significant effects to the flow regime are caused by the reclamation and dredging works for the Town Quay to Mayflower Park development within Southampton Water and the Cowes Breakwater in the Central Solent;

The effects of the Berth 201/202 redevelopment on the flow regime is shown to be minimal, however it is possible it may have slightly greater significance if the Mayflower Park development did not take place;

The Mayflower Park reclamation causes a deviation of estuary flows creating areas where peak flood flow speeds are both increased and decreased by up to 0.1m/s, a change approaching 50% in these areas. Flows over the shallow subtidal and intertidal are changed less, generally an increase of 0.02m/s (circa 10% of baseline flows). On the ebb, a similar flow diversion occurs with increased flows generally up to 0.08m/s, equivalent to about 20% of the existing baseline flows. These increases are up to double the decrease that occurred from the approach deepening alone;

The additional developments cause an additional marginal lowering of the estuary SSC by about 0.2mg/l (circa 10% of that arising from the approach dredging alone);

The Mayflower Park development will increase sedimentation rates by up to about 0.05m/year in the immediate lee of the works which will increase maintenance dredging in the berths of the Western Docks and potentially create a new requirement across the entrance to Town Quay Marina and the marina itself; and

Sedimentation in the new 201/202 Berth will be increased over the present between 0.01-0.02m/year, however this does not include redistribution of sediment from the disturbance to the pocket from vessels. The maintenance dredging commitment is therefore expected to be higher than indicated by the modelling.


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References ABPmer, 2008. Modelling of an additional breakwater and Shrape Marina - Cowes Harbour. Report to South East England Development Agency and Cowes Harbour Commission, ABPmer Report R1411. Burt, N., Land, J.M., and Otten, H., 2007. Measurement of Sediment Release from a grab dredger in the River Tees, UK, for the calibration of Turbidity Prediction Software. Proceedings of WODCON XVIII, paper 7C-3, pp 1173-1190. Kirby, R. and Land, J.M., 1991. The impact of dredging - A comparison of natural and man-made disturbance to cohesive sedimentary regimes. CEDA-PIANC Conference - Proceedings of Accessible Harbours, Session B, Paper 3. Pennekamp, J.G.S, and Quaak, M.P., 1990. Impact on the environment of turbidity caused by dredging. Terra et Aqua, 42, pp 10-20. Pennekamp, J.G.S., Epskamp, R.S.C., Rosenbrand, W.F., Millie, A., Wessel G.B., Arts, T. and Deibl, I.K., 1996. “Turbidity caused by dredging; viewed in perspective”. Terra et Aqua, 64, pp 10-17.

Upper: Extent of local model Lower: Extent of regional model

Figures C1- C2

1°

45' W

1° 3

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1°

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1°

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50° 45' N

51° 0' NN

Depth (m ODN)Above 0

-10 - 0-20 - -10-30 - -20-40 - -30-50 - -40-60 - -50-70 - -60-80 - -70

Below -80

Wate

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Current speed and direction calibration at Lains Lake (spring tide), local model

Figures C4a-b

Suspended Sediment Calibration at Fawley and Lains Lake (spring tide), local model

Figures C5a-b

Sediment deposition patterns, local model Figure C6

Water level calibration on spring tides, 2D regional model. Site A, C, E are Dock Head, Portsmouth and

Sandown respectively Figures C7a-c

Current speed and direction calibration, 2D regional model – Site I Figure C8

Baseline and scheme tidal elevation up estuary Figure C9

Baseline and change (scheme-baseline) in flood flow speeds (m/s), HW -1.5 hours at Dock Head

Figure C10

Baseline and change (scheme-baseline) in ebb flow speeds (m/s), HW +4.3 hours at Dock Head

Figure C11

Location of time series points Figure C12

Time series of water levels, current speed and

direction at Berth 206 Figure

C13


direction at Channel 8 Figure

C14


direction at Channel 10 Figure

C15

Baseline and change (scheme-baseline) in SSC during

peak flood flows Figure

C16

Baseline and change (scheme-baseline) in SSC at HW Figure C17

Baseline and change (scheme-baseline) in SSC during

peak ebb flows Figure

C18

Baseline and change (scheme-baseline) in SSC at LW Figure C19

Time series of SSC at Berth 206, Channel 8 and

Channel 10 (spring-neap cycle) Figure

C20

Annual background and change (scheme-baseline) in

sedimentation Figure

C21

Time series of sedimentation at Berth 206, Channel 8

and Channel 10 (spring-neap cycle) Figure

C22

Maximum SSC and sedimentation during dredging of Berth 201/202 (Upper Swinging Ground)

Figure C23

Time Series of SSC and sedimentation at Channel 08 during dredging of Berth 201/202

Figure C24

Time Series of SSC and sedimentation at Berth 206 during dredging of Berth 201/202

Figure C25

Time Series of SSC and sedimentation at Channel 10 during dredging of Berth 201/202

Figure C26

Time Series of SSC and sedimentation at Marchwood Moorings during dredging of Berth 201/202

Figure C27

Maxim

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28

Maximum distribution of SSC and sedimentation arising from disposal at the Nab Deposit Ground over

a spring-neap cycle. Locations of time series and transect shown

Figure C29

Time series of SSC and sedimentation at Site 11 during disposal at the Nab Deposit Ground

Figure C30


Figure C31


Figure C32

Figure C33


Figure C34

SSC along sections at peak ebb following disposal. Transect axis shown in Figure C29

Sedimentation along sections at HW following disposal of material. Transect axis shown in Figure C29

Figure C35

Figure C36

Time series of SSC averaged over different depths (Sites 11 and 5 – Figure C29)

Map plots of WL change for HW and LW (In-combination minus Southampton Approach

Channel Dredge Scenario) Figure

C37

Map plot of WL change for HW and LW

(Southampton Approach Channel Dredge-baseline) Figure

C38

Estuary wide change in Mean Spring Flow Speeds (In-Combination minus Southampton

Approach Channel Dredge Scenario) Figure

C39

Change in Mean Spring Flow Speeds – Redbridge to Dock Head (In-Combination minus

Southampton Approach Channel Dredge Scenario)

Figure C40

Baseline and changes to flood flow speeds (m/s) due to Southampton Approach Channel Dredge, HW -1.5

hours at Dock Head

Figure C41

Baseline and Changes to ebb flow speeds (m/s) due to

Southampton Approach Channel Dredge, HW +4.3 hours at Dock Head

Figure C42

Change to sedimentation patterns (In-Combination minus Southampton Approach

Channel Dredge Scenario), m/year Figure

C43

Change to sedimentation patterns Redbridge to Hythe (In-Combination minus Southampton

Approach Channel Dredge Scenario), m/year Figure

C44

Annex C1 Southampton Approach Channel Dredge - Model Calibration


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Annex C1. Southampton Approach Channel Dredge - Model Calibration Summary This report provides details of the set-up and calibration of a combination of modelling tools established for the assessment of potential scheme-related impacts for the proposed channel deepening to Southampton Water and Approaches. The models described have been designed to enable the proposed scheme to be represented together along with consideration of other proposed developments, which will also need to be included as part of the investigation of in-combination effects. A brief background on key physical processes within the Solent and Southampton Water is provided which are of relevance to model calibration. A description of the purpose of each model is provided together with a quantitative assessment of the models ability to reproduce observed conditions. Where possible qualitative assessments are also included to provide increased confidence in the models ability to represent the baseline conditions. The suitability of each model is finally assessed against its intended purpose.


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1. Introduction ABPmer has been commissioned by ABP Southampton to undertake Environmental Impact Assessments (EIA) for the proposed approach channel deepening within The Solent and Southampton Water as well as the 201/202 Berth Works, as identified in Figure 1. As part of the EIA, a suite of numerical models has been configured to help establish the magnitude and extent of changes to the physical marine environment. Details relating to the configuration, calibration and validation of these models are provided in this report. Effects associated with the proposed channel deepening have the potential to affect coastal processes within Southampton Water and The Solent. The extent and manner of these effects is complex and difficult to predict. Consequently, a numerical model programme is required to allow any effects to coastal and estuarine processes associated with the proposed channel deepening to be identified and, where possible, quantified, for the scheme in its proposed configuration, during dredging and taking account of possible in-combination effects with other likely proposals. The suite of models developed to inform the Environmental Impact Assessment (EIA) allows investigation of the following aspects associated with the works: Short-term changes in hydrodynamic and sedimentary regimes; Sediment plume dispersion during dredging operations; Dispersion of dredged material from disposal sites; Long-term morphological response within Southampton Water; Potential for ‘in-combination’ effects due to interaction with other proposed

developments including any identified beneficial uses, particularly within Southampton Water;

Future maintenance dredging requirements; and Any modified response due to climate change.

These models have been set up with the aim of assessing stakeholder concerns, as identified during the consultation process.

2. Study Area The seaward extent of the study area lies beyond the Solent since there is a requirement to investigate the effects of deepening of the Nab Channel and disposal at the Nab site. The regional model extends into the English Channel to ensure that the potential excursion of fine sediments is contained within the model. The near-field (local) model extends into the Solent to allow environmental impacts resulting from the proposed deepening within Southampton Water and the Thorn Channel to be studied in detail. The complex tidal features observed within the Solent result from its central location within the English Channel. Tidal conditions within the Solent are directly influenced by the close proximity of an amphidromic (nodal) point within the tidal system. Tides within the Solent typically exhibit a characteristic mid-tide stand as well as a double high water (HW), features,


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which are most obvious within Southampton Water. These features of the tide result in a two-stage flood tide and an extended duration of HW. The models applied at both scales therefore need to take full account of these features to enable realistic predictions of the tidal regime. A more detailed explanation of the tidal characteristics within Southampton Water can be found elsewhere (ABPmer, 2007a).

3. Physical Processes 3.1 Vertical Mixing

Previous investigations have successfully reproduced sediment transport processes within Southampton Water using a two-dimensional approach (ABP Research, 2000). However, the suitability of a 2D modelling approach has been re-assessed. The estuary number, E, can be used as an indication of the degree of vertical mixing within an estuary. For average freshwater flow conditions, the Southampton Water estuary number ‘E’ varies from 0.34 on spring tides down to 0.03 on neaps. A value of E greater than 0.09 indicates progressively well-mixed conditions (Ippen, 1966), which confirm that in general, the estuary is vertically well-mixed. However, under neap tide conditions with high river flows from the River Test, it is possible to have localised stratified conditions with the lower density freshwater overlying the denser saline water within the estuary. This effect is largely confined to the upper reaches of the estuary near the Bury Swinging Ground. To establish the extent of vertical flow structure within Southampton Water, measured vertical velocity profiles were analysed from available field datasets. This analysis was able to confirm that for most of the estuary there is little evidence of any significant variation in hydrodynamic conditions through the water column.

3.2 Flow Patterns

The conceptual understanding of sediment transport processes within Southampton Water (ABPmer, 2007a) highlights the importance of a reversal of the currents over the intertidal zones with the onset of the Young Flood Stand and HW. The model is required to demonstrate that this phenomenon is reproduced. The mechanism by which this occurs results from a combination of the plan-form of the channel and the inertia of the flooding tide. The flooding tide experiences a constriction in the upper reaches of Southampton Water resulting in a local increase in tidal range and a gradient in water levels up the estuary. Initially this is balanced by the inertia of the flooding tide. As the flood currents subside with the onset of the Young Flood Stand and HW, the water level gradient overcomes this inertia. This occurs first over the intertidal zones because of the increased friction the floodwaters experience here. The patterns of the reversed flows are complicated by the shape of the estuary in the vicinity of Dock Head, Fawley, the entrance to the River Hamble and Calshot Spit. This flow reversal is thought to have an important influence on sedimentary processes within Southampton Water and should therefore be resolved by any model attempting to reproduce such processes.

3.3 Sediment Supply

The primary source of sediment to Southampton Water is the fine-grained suspended sediment within the Solent, which mostly originates from the background sediment load within the


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English Channel. Spring tides and the unique tidal characteristics draw sediment into the estuary and enable deposition of this fine material throughout. The fluvial input of sediment is of second order importance amounting to approximately 5% of the supply from the Solent (ABPmer, 2007a).

3.4 Erosion and Deposition

Strong ebb currents, resulting from tidal asymmetry, lessen the net deposition of imported sediments in the middle and lower estuary. Ship wash is thought to contribute to erosion of the Hythe Marshes and Weston Shore cliffs. Cliff erosion is estimated to be an order of magnitude less than fluvial input (ABPmer, 2007a), which itself is small compared to the primary input from the Solent. Locally generated wind-waves result in further enhanced coastal erosion, particularly along the Weston Shore cliffs as a result of the prevailing wind directions.

3.5 Summary

The flow reversal associated with the Young Flood Stand (the period when tide levels remain approximately constant around mid-tide level on the flood) and HW provides a mechanism by which sediments can be transported into the intertidal zone. Current shear between the flows in the main channel and intertidal zones result in the mixing of sediment across the width of the estuary.

The patterns of long-term sedimentation in Southampton Water are determined mainly by the strong ebb dominance of the estuary and, in part, by the reverse flows observed over the intertidal areas.

4. Modelling Approach The modelling approach adopted combines existing models together with new models, which enables key processes to be investigated in more detail. The models are underpinned by the latest available datasets used during the model configuration (including the most recent multi-beam surveys) and proving (calibration/validation). Where appropriate, more than one model has been applied to provide an independent means of validating predicted changes, thus ensuring an increased level of confidence in the results. This approach has been developed in line with current best practice recommended in the ‘The Estuary Guide’ (as endorsed by the EA and Defra), which provides an overview of how to identify and predict morphological change within estuaries. The guide can be accessed through the website: http://www.estuary-guide.net/ and was developed under a current Defra and Environment Agency Flood and Coastal Erosion Risk Management R&D Programme (FD2119) which brings together all the existing and new outputs from the Estuaries Research Programme (ERP). The numerical modelling approach is designed to consider impacts relating to the proposed scheme at both regional and local scales. The regional scale model uses the DHI MIKE21 modelling suite and is used to assess effects at the Nab Channel and the adjacent deposit ground. An assessment of tidal excursions in this area was used to determine the minimum


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extent required to investigate sediment plume dispersion. The regional model is also required to consider potential in-combination effects that the proposals have with other plans or projects. Both regional (far-field) and local (near-field) scale models are required to consider tidal and sediment transport processes. At the regional scale (within the Solent), sediment transport involves mostly non-cohesive sediments and at the local scale (Southampton Water) cohesive sediments. The treatment of the two distinct sediment types is based on the predominant sediment found in each area although it is acknowledged that the sediment regimes are not exclusive since the Solent provides a source of fine sediments into Southampton Water. The regional and local modelling was undertaken using a two-dimensional (2D) approach, as required to provide a description of the relevant physical processes. At the local scale it was also possible to apply a one-dimensional (1D) model providing an independent means of assessing short and long-term changes in water levels along the estuary. Figure 2 provides a schematic representation of how the various models have been applied.

5. Regional 2D Model Design 5.1 Purpose

The regional model is required to establish the potential construction impacts of dredging of the Nab Channel as part of the proposed development. The model is also required to determine the fate of dredged material from the capital dredge and ongoing maintenance dredge material deposited at the Nab disposal site. The extent of the model means that it is also best suited to the consideration of potential cumulative impacts from the proposed scheme in-combination with other planned developments.

5.2 Software The 2008 version (Service Pack 1) of the MIKE21 from the MIKEZero software was used to set-up and operate the two-dimensional (depth-averaged) regional model. The MIKE21-HD (Hydro-Dynamic) and MIKE21-ST (Sand-Transport) modules were applied to investigate tidal and sedimentary processes at a regional scale. MIKE21-ST is potential transport model for non-cohesive sediment types. The term ‘potential’ is important here since the model evaluates transport rates assuming an infinite supply of sediment without accounting for the resulting changes in bed morphology. Despite these limitations the model is capable of providing a realistic representation of the regional scale transport processes using pre-defined hydrodynamic conditions, as derived during calibration of the tidal model.

5.3 Model Layout 5.3.1 Extent

The regional model is a dynamically nested grid within an established model of the entire English Channel. This model was most recently used in a study of surge propagation within the Solent region (ABPmer, 2004). The English Channel model is required to provide reliable forcing conditions for the Solent region where there are complex tidal interactions. The west


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and east boundaries of the regional grid are located at longitude 1°50’ W and 0°15’ W with a southern boundary 30 km south of the Isle of Wight, aligned approximately with latitude 50°20’ N, as shown in Figure 3. The regional grid extends into Southampton Water (including the Rivers Itchen, Hamble and Test) and the harbours of Portsmouth, Chichester and Langstone. The southern and eastern boundaries of the model are located some 31 km and 46 km respectively from the Nab disposal ground, as required to ensure that any sediment plumes from the dredging operations or disposal are mostly contained within the regional grid. The regional model was also required to include other proposed developments that may need to be considered as part of a cumulative impact assessment.

5.3.2 Resolution

By using three dynamically nested grids it was possible to refine the resolution of the grid in the area covering the proposed development and disposal site. Such localised refinement provides an enhanced representation of the bathymetry and therefore allows spatial variability in currents to be more accurately represented. The English Channel model has a grid resolution of 1200m by 1200m and includes two levels of nested grids for the regional area having a resolution of 400m and 133m. The finest resolution of 133m was required to allow the modifications to the Nab Channel to be represented in the model.

5.4 Bathymetry

The model bathymetry was derived from a compilation of contemporary surveys as detailed in Table 1. Table 1. Model bathymetry data

Description Source Southampton Water and the Nab ABP Southampton Water Approaches ABP River Itchen ABP River Medina ABP West Solent bathymetry UKHO East Solent bathymetry UKHO Beach profiles CCO Intertidal LiDAR (Southampton Water, Portsmouth and Chichester harbours) CCO River Hamble Hamble Port Authority English Channel SeaZone Fairsheet chart soundings: K8670, K8672, K9272, M1305, M1462, M1471, M1531_1, M1531_2, M1557, M2629 UKHO

The coverage of the various bathymetry datasets is shown in Figure 4. Data from all sources was converted to a common ‘XYZ’ format. The X and Y values refer to easting and northing coordinates based on the OS reference grid for Great Britain (OSGB). The Z value is the bed level, which was converted from the spatially varying chart datum to a common reference datum of Ordnance Datum Newlyn (ODN), as necessary. Interpolation routines were then applied to translate the data onto the regular model grids. The most recent survey data were


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interpolated first and gaps were then filled using predated survey data, LiDAR data, fair sheet data and finally, the hydro-spatial data from SeaZone.

5.5 Boundary Conditions

The English Channel model includes tidal forcing from time-varying water levels applied at both the eastern and western limits of the model. These water levels were derived from harmonic constituents provided by the Proudman Oceanographic Laboratory (POL) from their CS3 (Continental Shelf) model. The English Channel model has been configured to simulate tidal conditions for September/October 2007 including a full spring-neap cycle. Fluvial inputs from rivers were not included in the regional model due to the dominance of tidal processes at this scale.

5.6 Parameter Settings

The key parameters applied in the model are listed in Table 2. A map of spatially varying bed friction (Figure 5) was used to make fine adjustments during the model calibration.

Table 2. Regional hydrodynamic model parameter settings

Parameter Value or Range Units Time-step 30 secs Bed Friction (space varying Manning number) 17-40 m1/3/s Eddy Viscosity (Velocity based Smagorinsky formula) 0.5 m2/s Flooding depth 0.3 m Drying depth 0.2 m

A depth-dependent variation in roughness values was found to provide the best representation of tidal propagation and peak current magnitudes throughout the model domain. Within MIKE21 high Manning roughness values relate to low physical roughness. Manning values varied from 40m1/3/s in deep water channels to 20m1/3/s on intertidal areas, where enhanced bed roughness was also applied to represent the higher bed resistance associated with vegetated areas. The lowest value of 17m1/3/s was applied locally along the boundaries of the model to suppress numerical instabilities that were otherwise experienced.

6. Regional (2D) Model Calibration and Validation

The numerical model was calibrated against measured water levels from operational tide gauges and predicted values for tidal diamonds obtained using the UKHO TotalTide software. The model has also been calibrated against available measured historical and tidal diamond current data throughout the model domain. The model was calibrated for a spring-neap cycle between 7 and 21 October 2007. The model was also run for a spring-neap cycle between 16 and 30 September 2007 for validation purposes.


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6.1 Data Calibration and validation of the regional model has been assessed using a combination of measured and synthetic datasets. The measured data were obtained at locations throughout the Solent. Measured data were supplemented with additional datasets based on astronomic predictions of water levels and currents using the TotalTide software. The locations of available data considered during the calibration and validation exercise are shown in Figure 6 with further details of the measured and predicted data provided in Tables 3 and 4, respectively.

The distinction between measured and predicted (or synthetic) datasets is important since it is accepted that there can be limitations associated with the accuracy of predicted datasets. Wherever possible, measured datasets, that have undergone suitable quality checks, have been used in preference to predicted data. The initial model calibration focussed on the tidal propagation by comparing the amplitude and phase of the tidal constituents from the model and the measured water levels. Tidal conditions within the Solent typically exhibit a characteristic mid-tide stand as well as a double HW, features, which are most obvious within Southampton Water. The ability of the model to replicate these features was also considered during model calibration. Table 3. Measured data used for calibration/validation

Site Ref. Parameter Site Name Easting (m) Northing (m) A T Dock Head 442696 109460 B T Calshot 448837 102194 C T Portsmouth 462243 100483 D T Chichester 474627 96754 E T Sandown 460086 83778 F T Lymington 434877 93528 G C East Solent 459643 96000 H C Nab 475110 77930 I C Medina 449813 96717

T Tides, C Currents

Table 4. TotalTide data used for calibration/validation

Site Ref. Parameter TotalTide1 Ref Site Name Easting (m) Northing (m) J T Nab Nab 474197 85813 K T Ryde Ryde 462331 93071 L C SN007E Brackelsham 478463 93529 M C SN007S Spoil Ground 477077 82221 N C SN005AL Outer Solent 470821 63562 O C SN005AB Warner Shoal 466109 92555 P C SN004H Hurst 431771 89089 Q C SN005A Lymington 435990 92452

1 Tidal prediction software from the UK Hydrographic Office T Tides C Currents


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When comparing model currents with predicted values from TotalTide, it should be noted that the model values are depth-averaged whilst TotalTide is representative of near-surface flows. In general higher flows can be expected at the surface with lower flows near the bed where flows are reduced due to frictional effects. For this reason TotalTide data should only be used to make a qualitative comparison.

6.2 Calibration

During model calibration, bed roughness was used as the primary calibration parameter. Calibration runs focussed initially on reproducing water levels and then tidal currents. The bed roughness was adjusted to achieve a best fit to the measured data at locations primarily within the Eastern Solent. For model calibration, differences between observed and model data are presented. For both levels and peak speeds, a positive value indicates that the model is over-predicting observed values. In terms of phasing, a positive value indicates that the model is ‘late’ and with a negative value the model is ‘early’. For directions, a positive value indicates that the modelled directions are rotated clockwise and negative values anti-clockwise relative to the measured values.

6.2.1 Water Levels

The calibration of water levels was undertaken for October 2007 at eight locations as identified in Tables 3 and 4. The coastal locations are based on fixed tide gauges, which have been accurately levelled and can therefore be referenced to a fixed datum. For modelling purposes Ordnance Datum Newlyn (ODN) is the preferred datum. Water levels from devices deployed within the harbour or further offshore cannot easily be levelled. Instead the data can be referenced to a mean water level before applying an appropriate correction for the location under consideration. Figures 7a-c show the comparison of modelled and observed water levels at three of the locations where measured data were available (sites A, C and E). Results are plotted for a three day period for spring tidal conditions and show that the general shape of the tide curve is well reproduced by the model although the double HW at Site A tends to be exaggerated. The model is shown to reproduce many of the subtle features of the tide and demonstrates good agreement with the measured data. There is generally a high level of agreement in terms of the timing and elevation of high and low waters. As a quantitative assessment of the calibration, the mean difference in the phasing and elevation of high and low waters are provided in Tables 5 and 6. This assessment confirms that the modelled levels are within an error margin of ±0.19m, or 5% of the mean spring tidal range of 3.8m at Southampton. On this basis the model is demonstrated to predict water levels to an acceptable degree of accuracy. In terms of phasing, the time of HW in the model is late by up to 11 minutes. Conversely, low waters (LW) are shown to be early with a time difference of approximately 10 minutes. Overall, the model-predicted water levels are demonstrated to be calibrated to an acceptable standard, particularly in view of the 10 minutes temporal resolution of the measured data.


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Table 5. Calibration of regional water levels against measured data

Level Difference Phase Difference HW LW HW LW Site Ref.

avg.(m) avg.(m) avg.(mins) avg.(mins) A -0.08 -0.06 -5 10 B -0.16 -0.10 -7 11 C -0.17 -0.08 -11 8 D -0.19 -0.08 5 11 E 0.02 0.02 2 7 F 0.05 0.03 -10 8

Notes. Values are average differences over a 14-day period.

Table 6. Calibration of regional water levels against TotalTide data

Level Difference Phase Difference HW LW HW LW Site Ref.

avg.(m) avg.(m) avg.(mins) avg.(mins) J 0.07 0.04 2 19 K -0.04 -0.19 -2 22

Notes. Values are average differences over a 14-day period.

Figure 8 shows the comparison of modelled and TotalTide predicted water levels at the Nab (Site J). Modelled and TotalTide water levels agree to within 0.19m. However phase differences are evident, particularly at LW when the model is 19 and 22 minutes late at site J and K, respectively. In view of the good agreement in water levels against observed data and the coarse temporal resolution (of ten minutes) of both the model and TotalTide data, the model water levels are calibrated to an acceptable standard.

6.2.2 Currents

The modelled and observed currents at Site I are plotted for comparison in Figures 9a-b. The assessment of the calibration of modelled current speed and directions against measured data at all sites is presented in Table 7. Since the dates of the measured data did not coincide with the dates of the model run, this comparison was undertaken by matching the observations to model output for a period with compatible tidal range. The tabulated information on current speed and direction shows that in general, peak current speeds and directions are well predicted by the model. Modelled and observed peak flood and ebb currents agree within 20% of the observed currents, except at Site G where modelled peak flood currents are underestimated by 0.20m/s, or 22%. Current directions are also in good agreement, typically to within ±10°. Given the shortcomings associated with the measured data, the level of calibration achieved is considered to be acceptable. Comparisons of modelled and TotalTide currents at sites M and O, for coincident dates, are shown in Figure 10a-d. Results are plotted for a three day period for spring tidal conditions. As a quantitative assessment of the calibration, the mean difference in the peak current speeds and directions are provided for all sites in Table 8. The largest difference in current speeds occurs at Site P where model values are less than TotalTide values by 0.26m/s on the ebb, which equates to 11% of the mean current speed magnitude. At all TotalTide sites considered, current directions are within 5° of observed values.


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Table 7. Calibration of regional currents against measured data

Current Speed Difference (m/s) Direction Difference (°) Site Ref. Flood Ebb Flood Ebb

G -0.20 (22%) -0.12 (12%) 4 -7 H - -0.08 (7%) - 14 I 0.06 (15%) -0.12 (12%) 4 -7

Notes. The values are given as a percentage of the observed speed in brackets. Values are based on one tidal cycle

Table 8. Calibration of regional currents against TotalTide data

Current Speed Difference (m/s) Direction Difference (°) Site Ref. Flood Ebb Flood Ebb

L 0 0.05 (10%) -1 0 M -0.08 (7%) -0.09 (8%) -3 5 N 0.14 (9%) 0.05 (3%) -2 -5 O -0.07 (8%) -0.09 (15%) 5 4 P -0.05 (2%) -0.26 (11%) 2 2 Q 0.03 (3%) 0.01 (1%) 5 -3

Notes. The values are given as a percentage of the observed speed in brackets. Values are average differences over a 14-day period.

6.3 Validation 6.3.1 Water Levels

Further validation of water levels predicted by the regional model was undertaken for a spring-neap cycle in September 2007 and the comparisons are presented in Figures 11 and 12. The comparison shows a similar level of agreement between TotalTide/measured data and model data as found during model calibration. The model is therefore considered to provide accurate predictions of water level variations within currently accepted standards relevant to coastal modelling applications.

6.3.2 Currents

The validation of current speeds and directions within the regional model was also undertaken for a spring-neap cycle in September 2007. Results are presented in Figures 13a-b and 14a-b. In terms of current speed magnitude, the discrepancies between the model and measured/TotalTide datasets are generally similar in magnitude to the values found during calibration. Site N is the only location at which the agreement between the model and TotalTide is notably poorer than during the calibration run. At this location the modelled spring current speeds are higher than those from TotalTide, while the modelled neap current speeds are lower than TotalTide. During the validation run, which corresponds to a period of larger springs, the difference between the modelled and TotalTide currents is exaggerated. The differences in peak currents have not been quantified as part of the model validation but are presented graphically to enable a qualitative assessment to be made. The differences between a depth-average and surface current (as predicted by TotalTide) mean that a quantitative comparison would be inappropriate.


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6.3.3 Sediment Transport

The model is required to reproduce representative transport patterns within the Eastern Solent to determine whether there are likely to be any significant changes to the sediment transport as a result of dredging associated with the widening and deepening of the approach channel. The main focus of this study is on the effects of the dredging of the Nab Channel.

A key input to the regional sediment transport model is a map of the distribution of grain-size data covering the model domain. These data were mapped based on particle size distribution data from available sediment grab samples. Samples were available in the vicinity of the Nab deposit site and also interpreted from the BGS sediment map for the region (BGS, 1990) of surficial sediment deposits. These two datasets were combined to provide a sediment map over the wider Solent region. The resulting map of grain size distribution, as applied in the model, is shown in Figure 15. It should be noted that the BGS grain size data was coarser (0.314 mm) than the grab sample data (with a mean of 0.18 mm). This difference reflects the different sampling methods, with the grab samples representing the finer surface sediments. For a conservative approach the measured grain sizes are used in the distribution map and the BGS data is only used to qualitatively describe the sediment distribution. The key parameters applied in the model are listed in Table 9. The Meyer-Peter and Muller formulation for bedload transport was used. The ST model was run for spring tides covering 6 tidal cycles (from HW to HW, between 11 and 14 October 2007). Table 9. Sediment model parameter settings

Parameter Value Units Gradation 1.83 - Critical Shields Parameter, θcr 0.06 - Water Temperature 10 °C

There is limited scope for the validation of such potential sediment transport models. The ultimate goal of this process was to provide a model which reproduces, at least in qualitative terms, the general sediment transport pathways. One of the most suitable and widely adopted approaches used in developing a conceptual understanding involves defining the key elements of the sediment budget within the area and identifying the controls on sedimentary processes. This approach is often qualitative due to the inherent problems associated with quantifying a sediment budget. There are a number of different elements of a sediment budget or regime that can broadly be divided into sources, pathways and sinks. Conceptual studies of the Solent sediment regime (Bray et al, 1998; ABPmer, 2007b) indicate that the primary sediment sinks in the Solent include Bramble Bank, Medmerry Bank, Ryde Sands and Princess Shoal. The modelled potential sediment transport vectors and areas of net sediment erosion and accretion are plotted in Figures 16 and 17, respectively which are shown to reproduce many features of proposed transport pathways. These plots also indicate that the model is capable of identifying the known areas of sediment accretion, providing confidence that the sediment transport model is adequate for investigating potential changes in sediment transport pathways.


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Modelled erosion and accretion trends within the Nab Channel are shown in detail in Figure 17b. For this section of the channel the model indicates net erosion. However, in the northern section of the channel there is net accretion. This is consistent with knowledge of the area where there is known to be a requirement for maintenance dredging.

The sensitivity of the potential sediment transport pathways to various model parameters was tested. In particular the model sensitivity to gradation, water temperature, the critical shields parameter, transport theory and to the model run period was tested. While these parameters had a quantitative effect on the results, the net transport trends and the areas of deposition/accretion were unchanged.

6.4 Summary

Overall, the regional tidal model is shown to be calibrated and validated to a high standard in terms of water levels, current speeds and directions, with the rotational characteristics of the tide and characteristic features of the Solent tides resolved by the model. In particular the model is proven to produce an accurate description of tidal currents at fixed stations. An independent validation of interpreted tidal streams at hourly intervals on a spring tide was undertaken before using this information to provide an update to charts presented in the tidal ‘Solent Tides’ publication (Bruce, 2004). The author of this publication confirmed that both the magnitude and direction of model predicted currents were in good agreement with observations throughout the model domain. In one area to the south-west of Hurst Point, the previously published direction of streams over The Shingles bank on the flood tide were modified to be normal to this feature, as predicted by the model. This direction was verified by field observations which involved drifting over the feature in a boat (Peter Bruce, pers. comm.). A qualitative assessment of the sediment transport model confirms that it is able to reproduce observed characteristics of sediment pathways. The sediment model is also able to reproduce channel infill in areas where maintenance dredging is known to be required. The high standard of calibration achieved throughout the majority of the model domain confirms that the model is suitable for the assessment of changes to both hydrodynamic and sedimentary regimes, providing these can be adequately represented at the resolution provided by the model.

7. Local 2D Model Design

7.1 Purpose The local model is required to provide a detailed assessment of potential scheme impacts within Southampton Water and the Thorn Channel in terms of hydrodynamic conditions and cohesive sediment transport. As part of the assessment the model is required to provided a 2D description of high and low water surfaces for the calculation of habitat gains/losses as an indirect consequence of the channel deepening.


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The model is also required to assess potential cumulative impacts from the proposed scheme in-combination with other planned developments within Southampton Water, particularly where these developments could not be resolved within the regional scale model.

7.2 Software Version 3.27 of the Delft3D-FLOW module, developed by Delft Hydraulics in the Netherlands, was used to configure the tidal model. This modelling system offers the flexibility of operating as a two-dimensional (2D), depth-averaged model or as a layered three-dimensional (3D) model. In this instance the model was applied in 2D mode, based on the findings of the review of vertical structure within the estuary. The FLOW incorporates a fully coupled sediment transport module that was used to simulate sediment transport processes. This sediment model was applied within the study to investigate changes in the sediment regime which considers the predominantly cohesive sediment found within Southampton Water.


The seaward boundaries extend sufficiently far into the East and West Solent to ensure that changes resulting from deepening of the Thorn Channel can be represented in the model. The optimised model domain is shown in Figure 18. Upstream of the road and railway bridges at Redbridge, the grid was artificially extended to enable the tide to propagate correctly within the upper reaches of Southampton Water.

7.3.2 Resolution The curvilinear grid arrangement chosen allows enhanced resolution of the channel to be provided with a lower resolution covering intertidal areas. Since the proposed scheme involves both widening and deepening of the channel in some areas, the lateral widening should ideally be resolved by more than one grid cell. For a typical widening of 80m, the lateral grid resolution of 40m is therefore equivalent to 2 cells.

7.4 Bathymetry

The bathymetry applied in the model was derived from a range of data sources. The primary datasets for Southampton Water are the 2007 ABP surveys and LiDAR from 2005. The coverage of the combined bathymetric and topographic datasets is shown in Figure 19. Table 10 identifies all of the bathymetric datasets used. The ‘grid cell averaging’ method was used to interpolate the data onto the model grid together with the ‘average depth’ interpolation option, followed by careful, manual, adjustment to ensure that narrow river channels can be represented by the grid. Minor changes to the interpolated model bathymetry were required near the boundaries to help improve model stability. Numerical stability problems were also encountered between Berth 207 and the Redbridge Channel due to the steep gradients in bed elevations. It was found necessary to reduce this gradient by modifying bed levels in order to maintain model stability.


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Table 10. Model bathymetry data

Description Source Southampton Water Channel ABP Southampton Water Approaches ABP River Itchen ABP River Medina ABP West Solent bathymetry UKHO East Solent bathymetry UKHO Beach profiles CCO LiDAR CCO River Hamble Hamble Port Authority


For reference, Table 11 provides the mean high and low water levels for spring tides, as defined by the UK Hydrographic Office (UKHO). The local model was configured to run for a 23-day period from 20 September 2007 until 13 October 2007. Boundary conditions for the Southampton Water model were based upon field data records from Lymington (device maintained by the New Forest District Council) and Bramble Bank (device maintained by ABP Southampton). Both data sets are available for download from the Channel Coastal Observatory website. Some minor modification of the field data was required to achieve calibration. The water elevations at both locations were lowered by 0.174m. The Bramble data set is advanced by 5 minutes to account for the spatial difference in the field data location and the orientation of the eastern boundary of the model. Additionally, the Bramble Bank HW levels were raised by 0.5% while LW were lowered by 6%. No further modification of the Lymington data was required before application to the west boundary. Table 11. UKHO water levels

Location MHWS (m OD) MLWS (m OD) Calshot 1.71 -2.05 mid - Southampton Water 1.71 -2.22 Dock Head 1.71 -2.31 Redbridge 1.80 -2.36 Hamble 1.81 -2.30 Itchen 1.84 -2.51

Average winter river flow rates (ABPmer, 2007a) and sediment concentrations were applied in the model to represent the three main tributaries to Southampton Water, as detailed in Table 12. Table 12. River boundary inputs

River Discharge (m3/s) Sed. Conc. (mg/l) Hamble 1.3 -* Itchen 11.9 1.5 Test 17.6 7.5 * Denotes no data available


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The time-dependent sediment concentrations applied to the east and west boundaries were derived using an empirical relationship. Sediment concentrations were varied as a function of tidal range with lower levels during neap tides and higher levels on springs. Figure 20 shows the time-varying sediment concentration levels as applied to the east and west boundaries.


Table 13 below provides key parameters as applied in the calibrated model. The hydrodynamic model was mainly calibrated using spatially constant value of Manning roughness equal to 40m1/3/s. Table 13. Local hydrodynamic model parameter settings

Parameter Value or Range Units Grid dimensions 309 x 285 - Manning bed roughness 40 m1/3/s Eddy viscosity 1.0 m/s2

Water density 1020 kg/m3

8. Local (2D) Model Calibration and Validation

The tidal model was calibrated against measured water levels at tide gauge locations throughout the model domain. An assessment of the errors in phase and amplitude was undertaken to enable the level of calibration achieved to be quantified. A similar assessment was then undertaken for tidal currents which are usually more sensitive to changes in local bed roughness. The calibration of a model is typically assessed for a sequence of spring tides with validation undertaken for neaps. During the validation stage, no further adjustments were made to the model parameter settings, as defined in Table 13. The period selected for calibration coincides with the peak spring tides occurring between 28

September 2007 and 30 September 2007. Validation of the model performance was assessed on neap tides between 4 October 2007 and 7 October 2007. A 15-day period beginning 28 September 2007 was considered for the assessment of sedimentary processes.

8.1 Data Modelled water levels were initially compared to water levels recorded at Calshot and Dock Head. Historic field measurements of current speeds and suspended sediment concentrations are available at other locations throughout the estuary and were used to assess the performance of the model throughout Southampton Water. Comprehensive dredge data are available from the dredging campaigns during the period autumn 2001. This data is used to calibrate sediment deposition rates within the middle and upper estuary. Sedimentary processes outside the zone of the dredge campaigns are assessed conceptually. The location of the field measurements are shown in Figure 21 with the coordinates of these sites provided in Table 14.


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Table 14. Location of field measurements

Site Name Easting (m) Northing (m) Calshot 449195 102394 Lains Lake 444882 107063 Line 42D 446015 106242 Line 48C 444735 107364 Itchen Entrance 443363 110979 Dock Head 442606 109496 Bury 438885 111654

8.2 Calibration

8.2.1 Water Levels Water level calibration was undertaken on spring tides between 28 September 2007 and 30 September 2007 inclusively. Table 1 presents the results of the calibration.

The statistical information provided in Table 15 confirms that at Calshot model predicted HW levels are on average lower than the field measured values. The model predicted tide levels at both Calshot and Dock Head are shown in Figures 22a-b. At Calshot it can be seen that the elevation of the first HW is well predicted by the model and the second HW is consistently under-predicted by the model. The analysis for Calshot HW therefore shows that on average these levels are under-predicted by 0.08m with a relatively large standard deviation of the same magnitude. Within the record provided in Figure 22a, the highest HW level based on the field data alternates between the first and second HW whereas the model shows the first HW to be the highest. The analysis of HW uses the field data to determine a single HW value and compares this with the nearest HW in the model predictions. This explains the apparently poorer level of calibration at Calshot when compared to Dock Head. The accurate reproduction of the double HW is dependent local shallow water effects and interactions thus further improvements to the model calibration could not be achieved through the adjustment of model parameters such as bed roughness. LW levels at Calshot are predicted to be on average 0.05m lower in the model than the field data. Key features of the tide curve such as the Young Flood Stand and the steep gradient of the ebb tide are shown to be accurately reproduced by the model. Table 15. Calibration of high and low water levels

Level Difference (m) Phase Difference (mins) Location HW LW HW LW

Calshot -0.08 (0.08) -0.05 (0.04) +7.2 (6.7) -3.4 (4.7) Dock Head -0.01 (0.04) -0.04 (0.01) +6.9 (5.6) -2.5 (2.7) Notes: Standard deviation of differences in brackets A negative sign for level difference indicates model values are lower and positive higher A negative sign phase difference indicates model values are early and positive late


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At Dock Head the model predicted HW values are on average within 0.01m of measured values with a much lower standard deviation of 0.04m compared to the 0.08m found at Calshot. Figure 22 shows that the double HW is evident but not as pronounced in the model as in the field data. The general shape of the tide curve is however well reproduced by the model.

8.2.2 Currents

The model predicted currents are compared against historic flow data to provide a further assessment of the model performance. Differences between the field data and the model will partly result from subtle differences in the tide curves for the field data and model conditions. Table 16 provided a quantitative measure of model performance compared to the field data during spring conditions.

The characteristics of the flooding tide (and more specifically the variability of the young flood stand) are responsible for the most significant differences between the model and field data. The level of agreement is consistently higher during the ebb due to the simpler shape of the tide curve during this phase of the tide. The additional complexity of the flow patterns at Bury and the entrance of the River Itchen results in increased differences between the model and the field datasets.

For the calibration of spring conditions, the modelled current speeds compare well with the field data at Lains Lake and Calshot. At Lains Lake current speeds are 0.04m/s lower than observed, approximately 6.4%, during the flood tide. Higher currents observed during the ebb tide are matched a little closer, 0.03m/s greater than observed near Calshot.

Flood currents modelled in the vicinity of the entrance to the River Itchen exhibit the greatest difference when compared to the field records. The complexity of the flow patterns here during the flood tide result in the model underestimating the peak flow by 0.17m/s. A similar observation is made at Bury where peak flows are underestimated by 0.10m/s. A typical criterion for the calibration of current speeds within estuaries suggests that model values should be within ±0.15m/s of measured values. Only one of the values in Table 16 falls outside this criterion which can be explained by differences in conditions during the field measurements and conditions as applied in the model.

Modelled current flows are presented in Figures 23a-d and demonstrate the relative performance of the model compared to field data throughout a tidal cycle. Table 16. Calibration of peak currents

Peak Flood Speed (m/s) Peak Ebb Speed (m/s) Location Model Field Diff. Model Field Diff. Calshot 0.50 0.52 -0.02 1.24 1.21 0.03 Lains Lake 0.59 0.63 -0.04 1.19 1.19 0.00 Itchen Entrance 0.34 0.51 -0.17 0.82 0.81 0.01 Bury 0.11 0.21 -0.10 0.27 0.26 0.01


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8.2.3 Sediment Concentrations Suspended sediment concentrations predicted by the model are compared to available field records as part of the calibration process. There is often a high degree of variability in such field measurements with concentrations varying by a factor of two or more under compatible tidal conditions. Such variability can be explained by differences in local mixing, dispersion and sediment supply, possibly as a result of meteorological conditions and it is therefore often only appropriate to assess model calibration in qualitative terms. The available suspended sediment concentration (SSC) data was however used to identify the conditions required to re-erode sediments deposited over the intertidal areas and central channel of the lower estuary during the spring ebb tide. This erosion of intertidal sediments can also be observed in aerial imagery, Figure 24, which appears almost as a plume within the water column. The input parameters to the sediment model were adjusted so that it was possible to reproduce this effect. The final values of the key sediment parameters are as defined in Table 17. A range of spatial values of the critical threshold for erosion were used to achieve some of the spatial variations, requiring higher values in the majority of the model domain but lower values over the intertidal areas. Model predicted and field measured SSC data are compared in Figures 25a-d at four locations throughout the estuary. At Calshot the general variability in concentration levels is well reproduced by the model although there is an apparent offset in the model data which has a mean value of approximately 60mg/l compared to 30mg/l for the measured data. Further into the estuary at Fawley there is shown to be a high level of variability in the field data which is not replicated by the model although mean concentration levels are similar at approximately 55mg/l. At Lains Lake mean concentration levels are reduced further to approximately 40mg/l and as noted previously, the model shows a lower degree of variability in SSC about a mean level. At Woolston and Bury the field data shows lower levels of variability in SSC levels with mean levels of approximately 30mg/l and 25mg/l at these respective locations. The mean levels predicted by the model are in good agreement with the measured data confirming that the gradient in mean concentration levels along the estuary is well predicted by the model. Table 17. Sediment model parameter settings

Parameter Value or Range Units Settling velocity 0.09 mm/s Critical threshold for erosion 0.3-0.04 N/m2

Critical threshold for deposition 0.08 N/m2

Dispersion coefficient 100 m2/s

Erosion coefficient 0.00003 kg/m2/s Figure 26 provides a presentation of the longitudinal distribution in suspended sediment concentration levels along the estuary at peak flood. The plot shows that although concentration levels are generally uniform across the estuary, the model is capable of reproducing the raised concentration levels during the ebb tide when sediment is eroded from the intertidal areas along the southern shore. This process has been verified from aerial photography and provides additional confidence in the ability of the model to reproduce actual physical processes that occur in the estuary.


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8.3 Validation

8.3.1 Water Levels Model validation was undertaken on neap tides between 4 October 2007 and 7 October 2007. Table 18 quantifies the model performance for this period. The characteristics of the tide curve are simpler during the neaps which no longer exhibit the pronounced double HW. The model perform consistently well throughout the range of neap tide conditions.

Modelled water levels are on average 5cm lower than measured values at Calshot. The modelled high and lows water levels at Dock Head have a zero mean difference relative to the measured data. The 0.06m standard deviation indicates that differences are more significant at HW than LW. In terms of the phasing of high and low waters, the analysis shows that the timing of both high and low waters is early but on average still within a 10-minute margin. Given the shallower gradients of the neap tide curve relative to springs, phase errors can be expected to be greater. Table 18. Validation of high and low water levels

Level Difference (m) Phase Difference (mins) Location HW LW HW LW Calshot -0.06 (0.05) -0.04 (0.03) -9.1 (11.2) -4.4 (6.8) Dock Head 0.0 (0.06) 0.0 (0.03) -7.3 (20.3) -4.6 (7.3) Notes: Standard deviation of differences in brackets A negative sign for level difference indicates model values are lower and positive higher A negative sign phase difference indicates model values are early and positive late

Figures 27a-b compares the modelled water levels with those recorded in the field. The modelled tide at Calshot is consistently lower than the field data throughout most of the period although the shape of the tide curve well reproduced. At Dock Head the model is shown to provide an accurate representation of the field data. The model successfully reproduces the increased tidal range in the upper reaches of the estuary and the small, but appreciable, delay in the tide close to the head of the estuary compared to the mouth. The increased tidal range in the upper reaches of the estuary is an important feature of Southampton Water and is essential for the generation of observed flow characteristics.

8.3.2 Currents

Validation of the model currents has been undertaken at two locations in the central region of Southampton Water. The lower current speeds experienced during neap tides are difficult to reproduce within a numerical model due to the weaker astronomic forcing which results in lower current speeds. Table 19 provides a quantitative assessment of the model performance for both peak flood and ebb tidal conditions. Figure 28 shows that the modelled current speeds provide a reasonable description of neap tide currents. However, at mid to late ebb, when peak currents are normally expected to occur, the field data exhibits a drop in speed whereas the model produces a defined peak. As a consequence the modelled peak currents are greater than those found in the field.


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Table 19. Validation of peak currents

Peak Flood Speed (m/s) Peak Ebb Speed (m/s) Location Model Field Diff. Model Field Diff. Line 42D 0.35 0.25 0.10 0.42 0.32 0.10 Line 48C 0.27 0.31 -0.04 0.35 0.23 0.12

As discussed previously, the model will be required to reproduce qualitative features of the tidal flow regime such as the flow reversal over intertidal areas which occurs around the time of HW on spring tides. Figure 29 demonstrates this feature of the tidal regime is reproduced by the model and provides further confidence in the model performance.

8.3.3 Sediment Deposition

Suspended sediment concentrations were calibrated primarily to achieve the correct distribution in mean levels along the estuary. However, this does not necessarily imply that sediment deposition rates are sufficiently well predicted. Therefore, further validation of the sediment transport model was carried out by directly comparing predicted sediment deposition with estimates of actual values. These mean annual volumes were derived from bi-annual maintenance dredging records averaged for the years 2001 to 2007 for each of the designated dredge zones as identified in Figure 30. Table 20 includes modelled sedimentation rates together with the values derived from the records.

Table 20. Mean annual deposition

Dredge Zones Location Observed (m3/yr)

Modelled (m3/yr)

Difference (%)

0-10 Bury 50,000 48,600 -2.7 10-50 Main Dock Area 234,800 199,800 -14.9 51-70 Itchen Approaches 96,900 104,000 +7.3 71-79 Channel 40,500 46,100 +13.8 0-79 All Dredged Areas 448,400 404,200 -9.9

From an assessment of the variability in dredged volumes over the 7-year period, these were found to vary by ±60% from a long-term average including annual and seasonal variability. If the calibrated model is able to reproduce dredged volumes or depths within this range then the model can be confirmed as performing within the range of natural variability. The modelled values are based on a 15-day spring-neap cycle with the results scaled to represent annual figures. The model predicts deposited sediment volumes in the vicinity of Southampton Docks dredged areas within ±15% of estimated values. The largest discrepancy of 14.9% is associated with the ‘Main Dock Area’ between the upper and lower swinging grounds of the Western Docks. Volumes of sediment reaching the area of Bury swinging ground, as estimated by the model, are within 3% of the observed values. The modelling approach adopted takes no account of seasonal effects which will partly explains differences from the observed values. However, based on previous modelling experience, the 15% error margin demonstrates that the model has achieved a better than average level of calibration.


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Further validation of the sediment deposition rates predicted by the model is provided in Table 21. At three of the five locations the predicted rates of deposition are ±25% of observed values, considered to be a good level of agreement given the limitations of the data. The model does however significantly over-predict deposition rates within Ocean Dock which suggests that in reality there is enhanced disturbance of sediments, possibly as a result of propeller wash which would be expected in this area. Table 21. Deposition rates

Dredge Zone(s) Location Observed (m/yr)

Modelled (m/yr)

Difference (%)

8 Bury swinging ground 0.313 0.2 -36.1 15, 18 Berth 201 0.195 0.2 +2.6

38, 39, 41 Junction Channel 0.224 0.18 -19.6 44, 46, 47 Ocean Dock 0.206 0.45 +118.4

58, 59 Lower Itchen Toe 0.323 0.4 +23.8

The levels of deposition predicted by the model are also presented in Figure 31 which clearly shows where the highest levels of siltation are experienced within the Southampton Docks and the wider estuary. Figure 32 shows a typical time-history of sediment deposition within the model at Calshot and Town Quay. At Calshot there is shown to be deposition occurring on neap tides with erosion during the following spring tides resulting in a net accumulation for the period considered. At Town Quay the model predicts a consistent accumulation over both spring and neap tides, as expected due to the reduced strength of the flows in this area. Information on sediment fluxes predicted by the model is presented in Figure 33. The plot shows the net flux of sediment for defined sections within the estuary over a 15-day period. In qualitative terms the model predicts a net import of sediment into the estuary along the main subtidal channels. Along the intertidal margins of the central region of the estuary there is predicted to be a net export of sediment although the magnitude of this flux is much less that the subtidal component. Whilst the general pattern of the predicted fluxes appears to be in agreement with our conceptual understanding of the system, the analysis of sediment fluxes will mainly be used to provide a baseline condition for the assessment of scheme related impacts.

8.4 Summary

Both water levels and currents a well predicted by the local model. Apart from reproducing conditions at fixed locations, the model is also shown to reproduce qualitative features of the complex flow patterns within the estuary such as the flow reversal over intertidal areas. The sediment model is shown to reproduce the general pattern in mean concentration levels along the estuary although the variability from this mean is less well predicted. Given the likely natural variability in peak concentration levels due to vessel wash and the stirring action of local wind-waves, for example, the calibration to mean sediment concentration levels is considered appropriate here providing this limitation is taken into account within any subsequent analysis of scheme impacts.


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In qualitative terms the sediment model provides a realistic representation of the distribution of sediment accumulation. The model is even shown to replicate the erosion of intertidal sediments as is known to occur during the spring ebb tides. Further, by applying a scaling factor to siltation rates for the 15-day simulation period, it is possible to predict mean annual siltation rates throughout the Southampton Docks to a reasonable level of accuracy. On this basis the sediment model is also considered to be suitably well calibrated for the assessment any scheme related impacts due to the proposed channel deepening within Southampton Water.

9. Local (1D) Model Design

9.1 Purpose The 1D hydrodynamic model is required to provide an accurate description of tidal water level variations within Southampton Water. The model thus provides an independent assessment of changes in water levels in response to the proposed channel deepening, including the influence of sea level rise. The more simplistic model formulation over 2D representation provides a key advantage that model run-times are much shorter and if required, the model can therefore be run for longer timescales. Predicted changes in water levels from the model are used further to assess changes in the exposure of intertidal habitats.

9.2 Software The 2007 version of MIKE11 (Service Pack 1) from the MIKEZero software was applied within this study. The modelling investigation required application of the hydrodynamic module of the software.


The 1D model of the study area was based on a series of cross-sections that extend from the open tidal boundary just seaward of Calshot Spit to the tidal limits of the River Itchen at Woodmill and River Hamble at Curbridge. An additional tidal boundary was applied near Redbridge at the downstream limit of the River Test. The model layout is shown in Figure 34. Examples of typical cross-sections from the model are shown in Figures 35a-b.

9.3.2 Resolution

The 1D model comprises a series of linked cross-sections which, if required, can be defined at irregular intervals. A constant spacing of 100m was applied between sections with a reduced spacing used in areas where the channel alignment changes rapidly. The model layout thus provides the necessary detail and still remains computationally efficient.

9.4 Bathymetry

The same bathymetry datasets as applied in the local model were used to define the cross-sections represented in the model, as detailed in Table 10.


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The model boundary conditions applied at the seaward limit were derived from water level time-series obtained from the tide gauge at Calshot Spit. Boundary conditions at the tidal limit of the rivers Itchen and Hamble were derived from freshwater discharge data obtained from the Environment Agency (EA), as described in Table 12. A time-series of water levels was derived from measured data at the Berth 206 tide gauge and applied to the tidal boundary at Redbridge.


For a 1D hydrodynamic model, bed friction is the only parameter available for calibration. Within the model a constant Manning’s roughness coefficient of 38.5m1/3/s is applied throughout, except for the upstream boundary at Redbridge. where. At this location a high roughness was required to damp numerical instabilities and a coefficient of 100m1/3/s was therefore applied.

10. Local (1D) Model Calibration and Validation

Due to the simplified representation of the estuary within a 1D model, it is only appropriate to calibrate against water level data. Whilst an average flow velocity for each cross-section can be obtained from the model, calibration against field measured currents should be restricted to reaches of rivers where there are no two-dimensional flow effects. No calibration of current velocities was undertaken since the purpose of the model was to provide predictions of water levels.

10.1 Data The calibration of water levels was undertaken at Calshot and Dock Head using measured field data from the locations identified in Table 14.

10.2 Calibration

10.2.1 Water Levels Comparisons were made for water surface elevation over a spring-neap tidal cycle. In each case an assessment of the difference in elevation and phase was calculated to compare model predictions against the measured data. Table 22 shows the comparisons for water surface elevation between the differences in the timing of high and low water (averaged from a spring and neap tide). Figure 36 shows the comparison between modelled and measured water levels over a spring-neap cycle tides at Calshot. Figures 37a and 37b provide a more detailed presentation of the tide curves on spring and neap tides respectively. Similarly, Figure 38 shows tidal conditions at Dock Head followed by single tide plots in Figures 39a and 39b.


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Table 22. Calibration of High and Low Water levels

Level Difference (m) Phase Difference (mins) Location HW LW HW LW Calshot 0.01 -0.04 -2.5 1.0

Dock Head 0.04 -0.06 -3.5 2.0

The results from the calibration show that the model compares favourably with the measured water levels as differences lie within ± 0.06m. There is also a good agreement between the observed and predicted timings of high and low water, with mean differences of within ±4 minutes, which is less than the temporal resolution of the tide gauge data which was available at 12-minute intervals. Differences observed between the model predictions and the measured data are likely to be due, at least in part, to meteorological effects which cannot be included within a 1D model.

10.3 Validation

10.3.1 Water Levels Figure 40 shows the comparison between model results and measured data at Dock Head for an alternative two-day period. The results from this validation exercise are summarised in Table 23 based on differences in predicted timings and elevations of high and low water. Table 23. Validation of High and Low Water levels

Level Difference (m) Phase Difference (mins) Location HW LW HW LW Dock Head 0.06 -0.06 2.0 6.0

The difference in high and low water levels is within the same error margins as found during calibration whilst there is a slightly increased average phase error for LW of 6 minutes compared to the previous range of ±4 minutes. Even so the calibration of water levels in the 1D model is as good if not better than the 2D local model.

10.4 Summary The results of the calibration and validation exercise for the 1D model confirm that the numerical model performs well in terms of the prediction of tidal water levels at the locations considered. The minor differences between the measured and modelled data can be explained by meteorological effects which cannot be included in such simplistic models.

11. Conclusions The calibration and validation of three independent models has been described in detail within this report. In this instance the models are required to assess impacts of the proposed channel deepening within Southampton Water and Approaches on the physical marine environment as


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part of the EIA process. An assessment of the suitability of each model for its intended purpose can be made from the conclusions provided in this section.

Regional 2D Model The regional 2D hydrodynamic model has been extensively calibrated and validated

against a combination of measured and predicted datasets. The model provides a good description of tidal water levels variations throughout the

Solent over a range of tidal conditions. The model provides a very good description of tidal currents throughout the model

domain and over a range of tidal conditions as demonstrated by the comparison at fixed positions.

Further validation of the model, not reported here, has confirmed that the model provides an accurate description of current patterns throughout a typical spring tidal cycle.

The non-cohesive sediment transport model has been demonstrated to provide a reasonable description of potential transport pathways and rates, within the limitations of such sediment transport models.

The model is suitable for the assessment of scheme related changes in both hydrodynamics and non-cohesive sediment transport processes.

Local 2D Model The local 2D hydrodynamic model has been calibrated and validated against available

measured datasets within Southampton Water. The model provides a good description of tidal water level variations although it does

not reproduce the second HW on spring tides as accurately as the first. The neap tide water level variations are more accurately reproduced by the model than

spring tides. Tidal currents and directions are well predicted by the model, even under the relatively

weak flow conditions experienced during neap tides. The model is also shown to be able to reproduce characteristic features of the tide

such as the flow reversal over intertidal areas which occurs around HW. The cohesive sediment transport model provides a good description of mean

concentration levels along the estuary although peak concentrations are less well predicted.

The model results from a 15-day simulation can be scaled to represent long-term deposition rates within areas where dredging within the Southampton Docks is undertaken.

The model also reproduces observed behaviour of sediments such as the erosion from intertidal areas near Fawley during the ebb tide.

The model is suitable for the assessment of scheme related changes in both hydrodynamics and cohesive sediment transport processes.


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Local 1D Model The local 1D model provides a highly accurate prediction of tidal water level variations

for both spring and neap tides. Both the first and second HW are well reproduced by the model. The model is suitable for the assessment of scheme related changes in water level.

The models described in this report will be required to examine changes between an existing baseline and developed scheme scenario. The contents of this report demonstrate that the baseline model has been calibrated to a certain level. Any residual error will remain in both baseline and scheme model configurations but will be eliminated when considering changes relative to a baseline condition. As an assessment tool the predicted changes can therefore be expected to have a higher level of accuracy than the baseline model calibration.

12. References ABP Research, 2000. Changes in the physical environment of Southampton Water TS/ME2. ABP Southampton: Dibden Terminal, Associated British Ports, Southampton, ABP Research & Consultancy Ltd,. Research Report No. R.770. ABPmer, 2004. An Improved Understanding of Surge Propagation Through The Solent, ABP Marine Environmental Research, Research Report R.1074. ABPmer, 2007a. A Conceptual Model of Southampton Water. Prepared for the Estuary Guide as a case study supporting document by I. Townend, available online at: <http://www.estuary-guide.net/pdfs/southampton_water_case_study.pdf> ABPmer, 2007b. East Solent Sediment Regime, ABP Marine Environmental Research, Research Report R.1393 (CONFIDENTIAL). ABPmer, 2008. Southampton EIA’s: Modelling Approach, ABP Marine Environmental Research Technical Note, March 2008. BGS, 1990. Wight - Sheet 50°N-02°W, 1:250,000 Series, Sea Bed Sediments and Quaternary Geology, British Geological Survey. Bray, M.J., Hooke, J.M., Carter, D.J. & Clifton, J., 1998. Sediment transport pathways, cells and budgets within the Solent. Solent Science conference, Southampton Oceanography Centre. Bruce, P., 2004. Solent Tides, Boldre Marine, ISBN 1-871680-05-0. Defra, 2006. Flood and Coastal Defence Appraisal Guidance FCDPAG3 Economic Appraisal; Supplementary note to operating authorities - climate change impacts. Ippen AT, 1966, Estuary and Coastline Hydrodynamics, McGraw-Hill, New York.

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Nab Nab

Berth 201/202

Extent of proposed dredging (green) through Southampton Water and out to the Nab Figure 1

Schematic Diagram of Modelling Approach Figure 2

Extent of Model Grids Figure 3

Coverage of Bathymetric Survey Data Figure 4

Manningroughness

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Roughness Map for Solent Regional Model Grid Figure 5

Location of Regional Model Calibration Data Figure 6

Water Level Calibration at Site A, Site C and Site E (Spring) Figure 7a-c

Water Level Calibration at Site J (Spring) Figure 8

Current Speed and Direction Calibration at Site I Figure 9a-b

Current Speed and Direction Calibration at Site M (neap tides) Figure 10a-b

Current Speed and Direction Calibration at Site O (spring tides) Figure 10c-d

Water Level Validation at Site E (Spring) Figure 11

Water Level Validation at Site J (Spring) Figure 12

Current Speed and Direction Validation at Site I Figure 13a-b

Current Speed and Direction Validation at Site M (spring tides) Figure 14a-b

Sediment Grain Size Distribution Map Figure 15

a) Potential Sediment Transport Pathways and b) Residual Current Vectors on Spring Tides Figure 16a-b

Predicted Erosion (orange) and Accretion (green)

Patterns for a) the Eastern Solent and b) Nab Channel

Figure 17a-b

Extent of 2D Local Model Grid Figure 18

Coverage of bathymetric and topographic datasets Figure 19

Time-varying Sediment Concentration Levels Figure 20

Location of Model Calibration Data Figure 21

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Current speed (above) / direction (below) calibration at Itchen Entrance (spring tide) Figure 23c

Current speed (above) / direction (below) calibration at Bury (spring tide) Figure 23d

Aerial view of intertidal erosion (indicated by dashed white arrows) at Fawley during the ebb tide Figure 24

(source: Google Earth)

Suspended Sediment Calibration at Fawley Figure 25a

Suspended Sediment Calibration at Lains Lake Figure 25b

Suspended Sediment Calibration at Itchen Entrance Figure 25c

Suspended Sediment Calibration at Bury Figure 25d

Erosion of sediment in model during ebb tide Figure 26

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Dredge zones for Southampton Docks and Approaches Figure 30

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Time-history of Sediment Deposition Figure 32

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:00

31/0

3/20

0600

:00

31/0

3/20

0612

:00

01/0

4/20

0600

:00

01/0

4/20

0612

:00

02/0

4/20

0600

:00

02/0

4/20

0612

:00

03/0

4/20

0600

:00

03/0

4/20

0612

:00

04/0

4/20

0600

:00

04/0

4/20

0612

:00

05/0

4/20

0600

:00

05/0

4/20

0612

:00

06/0

4/20

0600

:00

Dat

e/tim

e

Water elevation (m)

Dock

Hea

d tid

e gau

ge d

ata

Dock

Hea

d m

odel

wate

r lev

els

Wat

er L

evel

Calib

ratio

n at

Doc

k Hea

d (s

prin

g-ne

ap cy

cle)

Fi

gure

38

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

3

30/ 03/ 200617:00

30/ 03/ 200619:00

30/ 03/ 200621:00

30/ 03/ 200623:00

31/ 03/ 200601:00

31/ 03/ 200603:00

31/ 03/ 200605:00

31/ 03/ 200607:00

31/ 03/ 200609:00

31/ 03/ 200611:00

D ate/ t ime

D o ck Head t id e g aug e d at a

D o ck Head mo d el wat er levels

D if f erence

Water Level Calibration at Dock Head (spring tide) Figure 39a

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

3

26/ 03/ 200602:00

26/ 03/ 200604:00

26/ 03/ 200606:00

26/ 03/ 200608:00

26/ 03/ 200610:00

26/ 03/ 200612:00

26/ 03/ 200614:00

26/ 03/ 200616:00

26/ 03/ 200618:00

26/ 03/ 200620:00

D ate/ t ime

D o ck Head t id e g aug e d at a

D o ck Head mo d el wat er levels

D if f erence

Water Level Calibration at Dock Head (neap tide) Figure 39b

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

3

19/03/2006 00:00

19/03/2006 04:00

19/03/2006 08:00

19/03/2006 12:00

19/03/2006 16:00

19/03/2006 20:00

20/03/2006 00:00

20/03/2006 04:00

20/03/2006 08:00

20/03/2006 12:00

20/03/2006 16:00

20/03/2006 20:00

Date/time

Wat

er e

leva

tion

(m) O

DN

Dock Head tide gauge data

Dock Head model water levels

Difference

Water Level Validation at Dock Head Figure 40

Annex C2 Disturbance Parameters Backhoe Dredging of Stiff Clay (Greensand) from Upper Swinging Ground

Ann

ex C

237

54 B

erth

201

/202

Wor

ks

Mod

ellin

g D

redg

e D

ispe

rsio

n Sc

enar

io -

Lar

ge B

ackh

oe lo

adin

g B

arge

s -

Upp

er S

win

ging

Gro

und

Equi

pmen

t dat

aB

ucke

t Cap

acity

19m

3A

ssum

e w

ater

den

sity

1025

kg/m

3B

ucke

t Cyc

le T

ime

72se

cond

sA

ssum

e S

edim

ent d

ensi

ty26

50kg

/m3

Pro

duct

ion

Rat

e68

0m

3/ho

urm

3/da

y (2

4 ho

ur w

orki

ng)

No

Bar

ges

3B

arge

siz

esA

ve B

arge

In s

itu v

olum

eB

ucke

t cyc

les

Bar

ge F

ill R

ate

(Min

)C

alcu

late

d P

rodu

ctio

nD

eadw

eig h

Hop

per C

apac

ityD

ensi

ty k

g/m

3m

3to

fill

barg

em

3 in

situ

/hr

Load

che

ckV

ol d

raug

ht1

4750

2850

m3

1600

1873

9911

873

329

68.7

528

502

4750

2850

m3

1600

1873

9911

873

329

68.7

528

503

4750

2850

m3

1600

1873

9911

873

329

68.7

528

50

Bar

ge tr

ansi

t tim

e6.

59ho

urs

Ass

umed

Loa

ded

serv

ice

spee

d10

Kno

tsD

ista

nce

to D

epos

it32

.1N

aut M

iles

Dep

osit

Tim

e10

Min

utes

Bar

ge d

redg

er c

hang

e35

min

s

Dre

dge

Mat

eria

l:Fi

rm/S

tiff c

lay/

sand

B

rack

lesh

am B

eds

Ave

rage

Insi

tu D

ensi

ty19

00kg

/m3

Cha

ract

eris

tic g

rain

siz

e of

dis

turb

ed m

ater

ial

10m

icro

ns

Ass

umed

'S' F

acto

r bas

ed o

n K

irby

and

Land

(199

1)To

tal

12kg

(dry

sed

imen

t)/m

3 (in

situ

rem

oved

)

Pha

ses

of b

ucke

t cyc

le/ A

ssum

ed p

ropo

rtion

of t

otal

sed

imen

t dis

turb

ed

Buc

ket c

ycle

Mas

s di

stur

bed

Mas

s di

stur

bed

Rel

ease

rate

% o

f mat

eria

lB

ucke

t cyc

le

Pha

se ti

me

kg/m

3pe

r buc

ket c

ycle

kg/s

dis

turb

edP

hase

%(s

econ

ds)

kg1

Buc

ket l

ower

ing

5%

2518

0.6

11.4

0.63

2B

ed R

ippi

ng35

%25

184.

279

.84.

433

Wat

er C

olum

n W

ash

35%

2518

4.2

79.8

4.43

4S

lew

ing/

brea

king

wat

er s

urfa

ce25

%25

183

573.

17

C2.

1P

age

1 of

2

Mod

el S

imul

atio

n Sc

enar

ioA

nnex

C2

Leve

l in

Wat

er

Col

umn

Buc

ket L

ower

ing

Bed

impa

ct a

nd 'r

ippi

ng'

R

aisi

ng B

ucke

t S

lew

ing

Bar

ge lo

adB

arge

N

umbe

r

Buc

ket

cycl

es to

fil

l bar

ge

Bar

ge a

nd

dred

ger

Pos

C

hang

e tim

e (M

inut

es)

AS

urfa

ce- S

urfa

ce-1

m3.

17kg

/s1

9935

BTh

roug

hout

dep

th0.

63kg

/s4.

43kg

/s2

99C

Bot

tom

2m

4.43

kg/s

399

Rel

ease

Ti

me

18se

cond

s18

seco

nds

18se

cond

s18

seco

nds

Num

ber o

f cyc

les

per m

odel

sim

ulat

ion

Sim

ulat

ion

Leng

th0.

25D

ays

Bar

ge 1

Bar

ge 2

Bar

ge 3

No.

Dre

dge

cycl

es2.

352.

352.

35In

Situ

vol

ume

Dre

dge

4399

m3

4399

m3

4399

m3

Rel

ease

d se

dim

ent m

a53

tonn

es53

tonn

es53

tonn

es

Ke y

AS

lew

ing

BW

ater

col

umn

Was

h of

fC

Bed

Dis

turb

ance

Man

ual D

ata

Inpu

t

Spec

ific

Mod

ellin

g Sc

enar

io N

otes

1M

odel

to b

e ru

n w

ith d

eepe

ned

flow

dyn

amic

sus

ing

conv

ersi

on fa

ctor

for r

esul

ts b

etw

een

kg/m

^2 to

m o

f 135

kg/m

^3 (d

ry b

ed d

ensi

ty fr

om b

asel

ine

mod

el)

sedi

men

t pro

porti

es u

sed:

che

zy =

55

C2.

1P

age

2 of

2

Annex C3 Nab Disposal Parameters Backhoe Loaded Barges - Stiff Clay/Greensand from Upper Swinging Ground

Ann

ex C

337

54 B

erth

201

/202

Wor

ks

Mod

ellin

g D

ispo

sal D

ispe

rsio

n Sc

enar

io -

Bar

ge

Nab

Dep

osit

Gro

und

Mat

eria

l Sou

rce:

Upp

er S

win

ging

Gro

und

Equi

pmen

t dat

aH

oppe

r Cap

acity

2850

m3

Ass

ume

wat

er d

ensi

ty10

25kg

/m3

Dep

osit

Tim

e10

Min

utes

Ass

ume

Sed

imen

t den

sity

2650

kg/m

3Lo

aded

Dra

ught

4.8

met

res

Bar

ge L

oad

time

226

Min

utes

(Fill

and

ove

rflow

- ca

lc fr

om d

redg

e sp

read

shee

t)

Dre

dger

/Bar

geH

oppe

rA

ve H

oppe

rIn

situ

M

ass

of s

edim

ent r

elea

sed

Tota

l Rel

ease

Rat

eLo

ad D

ata

Dea

dwei

ghC

apac

ityD

ensi

ty k

g/m

3vo

lum

e m

326

72.4

2to

nnes

dry

sol

id/lo

ad44

54kg

/s fo

r10

Min

utes

4750

2850

m3

1600

1873

Bar

ge c

ycle

tim

e10

.27

hour

sA

ssum

ed L

oade

d se

rvic

e sp

eed

10K

nots

Dis

tanc

e fro

m d

redg

e l

32.1

Nau

t Mile

sD

epos

it Ti

me

5M

inut

esD

redg

e M

ater

ial:

Firm

/stif

f san

d/cl

ayP

re H

oloc

ene

Ave

rage

In s

itu D

ensi

ty19

00kg

/m3

Coh

esiv

e/N

on C

ohes

ive?

Non

coh

esiv

e/co

hesi

veD

eadw

eigh

t Cal

cula

tion

for v

olum

eLo

ad c

heck

2969

Cha

ract

eris

tic g

rain

siz

e of

dep

osite

d M

ater

ial m

ater

ial

Vol

to d

raug

ht m

arks

2850

Par

ticle

siz

e en

velo

peD

10(m

icro

ns)

D50

(mic

rons

)D

90(m

icro

ns)

Min

Max

Min

Max

Min

Max

01

312

1825

0

%C

lay

<2m

icro

ns%

Silt

2><

63 m

icro

ns%

San

d 63

><20

00%

>2

mm

Min

Max

Min

Max

Min

Max

Min

Max

1838

1060

060

05

11

mic

rons

25%

668

1114

29

mic

rons

55%

1470

2450

All

for

10M

inut

es3

60m

icro

ns20

%53

489

1

Not

e:M

odel

cha

ract

eris

tic s

izes

and

repr

esen

tativ

e pr

opor

tions

est

imat

ed fr

om p

artic

le s

ize

enve

lope

s of

dre

dged

mat

eria

l

Wat

er C

olum

n R

elea

se p

oint

for e

ach

char

acte

ristic

sed

imen

t

11

mic

rons

7030

779

kg/s

334

kg/s

29

mic

rons

5535

1347

kg/s

857

kg/s

All

for

10M

inut

es3

60m

icro

ns40

6035

6kg

/s53

4kg

/s

Mod

elle

d C

hara

cter

istic

sed

imen

t R

epre

sent

ativ

e pr

opor

tion

Cha

ract

eris

tic p

artic

le

Part

icle

siz

e

Rel

ease

Rat

e in

bo

ttom

2m

Mod

elle

d C

hara

cter

istic

sed

imen

t si

zes

% a

vera

ged

thro

ugh

wat

er c

olum

n be

neat

h %

rele

ased

in b

otto

m

2m o

f wat

erco

lum

nR

elea

se ra

te a

vera

ged

thro

ugh

wat

er c

olum

n

C3.

1P

age

1 of

2

Mod

el S

imul

atio

n Sc

enar

ios

(Com

pone

nt S

edim

ents

)

Mod

elle

d C

hara

cter

istic

sed

imen

t siz

e 1

Cha

ract

eri

stic

P

artic

le

size

(m

icro

ns)

A1

779

kg/s

Lo

aded

dra

ught

4.8

mB

Bot

tom

2m

133

4kg

/sR

elea

se

Tim

e

Tota

l dre

dge/

barg

e cy

cle

time

10.2

7H

ours

Dre

dge

Dep

osit

Tim

e10

.00

Min

utes

Mod

elle

d C

hara

cter

istic

sed

imen

t siz

e 2

Cha

ract

eri

stic

P

artic

le

size

(m

icro

ns)

A9

1347

kg/s

Lo

aded

dra

ught

4.8

mB

Bot

tom

2m

985

7kg

/sR

elea

se

Tim

e

Tota

l dre

dge/

barg

e cy

cle

time

10.2

7H

ours

Dre

dge

Dep

osit

Tim

e10

.00

Min

utes

Mod

elle

d C

hara

cter

istic

sed

imen

t siz

e 3

Cha

ract

eri

stic

P

artic

le

size

(m

icro

ns)

A60

356

kg/s

Lo

aded

dra

ught

4.8

mB

Bot

tom

2m

6053

4kg

/sR

elea

se

Tim

e

Tota

l dre

dge/

barg

e cy

cle

time

10.2

7H

ours

Dre

dge

Dep

osit

Tim

e10

.00

Min

utes

Key

AS

edim

ent a

dvec

ting

with

flow

as

falls

thro

ugh

wat

erco

lum

BS

edim

ent p

redo

min

antly

falli

ng to

bed

as

a de

nsity

cur

rent

and

then

bei

ng m

oved

on

by a

dvec

tion

and

bed

eros

ion

proc

esse

sM

anua

l Dat

a In

put

Spec

ific

Mod

ellin

g Sc

enar

io N

otes

1M

odel

to b

e ru

n w

ith e

xist

ing

flow

dyn

amic

s2

Insi

tu d

ensi

ty u

sed

1900

kg/

m3

how

ever

this

will

be

cons

ider

ably

low

er w

hen

dred

ging

sur

face

sed

imen

ts a

nd M

arch

woo

d se

dim

ents

Mod

elD

isch

arge

Lev

el

in W

ater

Col

umn

(met

res)

Dre

dger

/bar

ge d

epos

itA

vera

ged

in

Wat

ecol

umn

belo

w

Load

ed D

raug

ht (m

)

Ave

rage

d in

W

atec

olum

n be

low

Lo

aded

Dra

ught

(m)

Mod

elD

isch

arge

Lev

el

in W

ater

Col

umn

(met

res)

Dre

dger

/bar

ge d

epos

itA

vera

ged

in

Wat

ecol

umn

belo

w

Load

ed D

raug

ht (m

)

Mod

elD

isch

arge

Lev

el

in W

ater

Col

umn

(met

res)

Dre

dger

/bar

ge d

epos

it

C3.

1P

age

2 of

2

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